Corrige el error: "We did not find any section in your theme. A theme must have at least one b:section tag." En la siguiente plantilla:


<!DOCTYPE html>
<html lang="es" dir='ltr' xmlns='http://www.w3.org/1999/xhtml'>
<head>
    <title>Youtube Thumbnail Generator (High Quality Images)</title>
    <meta name='Description' content='YouTube Thumbnail Downloader automatically generates high-quality thumbnails or images from YouTube videos. Paste your YouTube video URL in the below box and click on "Download" button. Within seconds it will generate your sweet thumbnails.'/>
    <meta name='Keywords' content='YouTube Thumbnail Downloader,youtube, thumbnail, generator, screenshots, image generator, picture generator, maker, photo, creator,'/>
    <meta name='robots' content='index, follow'/>
    <meta name='revisit-after' content='2 days'/>
    <meta name='language' content='English'/>
    <meta name='geo.country' content='INDIA'/>
    <meta name='geo.placename' content='BIHAR'/>
    <meta name='distribution' content='global'/>
    <meta http-equiv='Content-Type' content='text/html; charset=UTF-8'/>
    <meta name='MSSmartTagsPreventParsing' content='true'/>
    <meta name='rating' content='general'/>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous"/>
    <script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
 
<b:skin><![CDATA[
   <style>
        /* CSS previamente establecido */
        #navbar-iframe { height: 0px; visibility: hidden; display: none !important; }
        html, body, div, span, applet, object, iframe, h1, h2, h3, h4, h5, h6, p, blockquote, pre, a, abbr, acronym, address, big, cite, code, del, dfn, em, img, ins, kbd, q, s, samp, small, strike, strong, sub, sup, tt, var, b, u, i, center, dl, dt, dd, ol, ul, li, fieldset, form, label, legend, table, caption, tbody, tfoot, thead, tr, th, td, article, aside, canvas, details, embed, figure, figcaption, footer, header, hgroup, menu, nav, output, ruby, section, summary, time, mark, audio, video {
            margin: 0;
            padding: 0;
            border: 0;
            font-size: 100%;
            font: inherit;
            vertical-align: baseline;
        }
        article, aside, details, figcaption, figure, footer, header, hgroup, menu, nav, section {
            display: block;
        }
        html {
            background: #eeeeee;
            text-align: center;
            line-height: 1;
            font-family: Georgia;
        }
        ol, ul {
            list-style: none;
        }
        blockquote, q {
            quotes: none;
        }
        blockquote:before, blockquote:after, q:before, q:after {
            content: '';
            content: none;
        }
        table {
            border-collapse: collapse;
            border-spacing: 0;
        }
        *:focus {
            outline: 0;
        }
        #hd {
            width: 100%;
            height: 390px;
            background-image: linear-gradient(to right, #0970eb, #008fef, #00a4d8, #00b3b6, #52be9a);
            margin: 0 0 50px 0;
            box-shadow: inset 0 -4px 10px -5px #000;
        }
        h1 {
            font-size: 24px;
            font-weight: bold;
            color: #fff;
            text-shadow: 1px 1px 1px #666666;
        }
        #hd p {
            max-width: 70%;
            margin: 20px auto;
            padding: 10px;
            background: #2A78C3;
            border: dashed #00C0C7 2px;
            font-family: Arial;
            font-size: 17px;
            font-weight: bold;
            line-height: 1.4;
            color: #FFFFFF;
        }
        #hd p span {
            color: #FFFF00;
        }
        input[type="text"] {
            width: 400px;
            height: 70px;
            padding: 0 0 0 10px;
            border: dashed #1a1a1a 2px;
            font-family: Georgia;
        }
        input[type="submit"] {
            height: 72px;
            background: #66bdff;
            background-image: linear-gradient(to bottom, #66bdff, #0d9eff);
            border: dashed #1a1a1a 1px;
            border-radius: 0;
            margin: 0 0 0 -2px;
            cursor: pointer;
            font-family: Georgia;
            color: #fff;
        }
        input[type="submit"]:hover {
            background: #f72525;
            background-image: linear-gradient(to bottom, #f72525, #b52222);
        }
        h2 {
            margin: 20px;
            font-size: 14px;
        }
        h3 {
            color: #A9A6A6;
            margin: 10px auto;
        }
        #hidden_div img {
            margin: 5px;
            border: solid #C8312B 3px;
        }
        #sorry {
            width: 200px;
            height: 30px; /* Corregido */
            margin: 10px auto;
            padding: 10px;
            background: #BA2020;
            border: dashed #7E1313 2px;
            color: #fff;
        }
        #iam {
            margin: 50px auto;
            color: #1a1a1a;
        }
        .footer-copy {
            margin: 50px auto;
            color: #c2c2c2;
        }
        .adsbox {
            padding: 2em;
            box-shadow: 0 5px 15px rgba(0, 0, 0, .16);
            border-radius: 5px;
            margin-top: 1em;
            background: #fff;
            text-align: left;
        }
    </style>

    ]]></b:skin>
</head>
<body class="text-center">
    <div id='hd'>
        <img alt='youtube thumbnail generator' src='https://1.bp.blogspot.com/-6iQK-ugxaMA/XzlEKT2IKCI/AAAAAAAAB8w/ch-bNMemZroo07x-raUTXatJH2jQZh1hACLcBGAsYHQ/s1600/aaaaaaa.png' title='High Quality YouTube Video Thumbnail Generator' />
        <h1>High Quality YouTube Video Thumbnail Generator</h1>
        <p>YouTube Thumbnail Generator automatically takes "High Quality" Screenshots (Thumbnails or Images) from YouTube videos. Paste your YouTube Video URL (<span>https://www.youtube.com/watch?v=zru82DBCIEw&amp;t=237s</span>) in the below box and click on "Download" button. Within seconds it will generate your sweet thumbnails.</p>
    </div>
    <div class="addthis_inline_share_toolbox centersocials"></div>
    <div class="adsbox container">
        Place your Ad Code 1     
    </div>
    <br /> <!-- Esta línea está correctamente formada -->
    <div class="container">
        <form onsubmit='showHide(); return false;'>
            <input autocomplete='on' id='technicalarpvid' placeholder='Paste A Youtube URL' type='text' />
            <input id='technicalarpvideoButton' type='submit' value='Download' class="btn btn-primary" />
        </form>
    </div>
    <div class="adsbox container">
        Place your Ad Code 2     
    </div>
    <div id='hidden_div' style='display:none'>
        <div id='thumbnails'>
        </div>
    </div>
    <script type="text/javascript" src="//s7.addthis.com/js/300/addthis_widget.js#pubid=ra-5c98f50a092025d4"></script>
    <footer style="margin-top: 30px; margin-bottom: 20px;">
        <div class='container text-center'>
            <div class='footer-copy'>
                <p>© Your Website Name <script>document.write(new Date().getFullYear())</script>
                <span>• </span>
                </p>
            </div>
        </div>
    </footer>

    <script type='text/javascript'>
        $(document).ready(function() {
            $('#technicalarpvideoButton').click(function() {
                function GetQueryParam(parameter, address) {
                    var p = escape(unescape(parameter));
                    var regex = new RegExp("[?&amp;]" + p + "(?:=([^&amp;]*))?", "i");
                    var match = regex.exec(address);
                    return match ? match[1] : null;
                }
                var videoURL = $('#technicalarpvid').val();
                var theID = GetQueryParam("v", videoURL);
                $("#thumbnails").empty();
                if (theID != null) {
                    $('<div></div>').html(
                        '<h2>I like you :) These are your thumbnails.. ** Click on a below image and Right Click on it at next Tab and Click "Save Image As..." to Download the image</h2>' +
                        '<h3>» Default Thumbnail</h3>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/default.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/default.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>' +
                        '<h3>» 3 Other Small Thumbnails</h3>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/1.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/1.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/2.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/2.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/3.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/3.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>' +
                        '<h3>» Medium Quality Thumbnails</h3>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/mqdefault.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/mqdefault.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a><br />' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/0.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/0.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>' +
                        '<h3>» High Quality Thumbnails</h3>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/hqdefault.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/hqdefault.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>' +
                        '<h3>» High Definition Thumbnail ( **Only available if, your video is more than 720p )</h3>' +
                        '<a href="http://img.youtube.com/vi/' + theID + '/maxresdefault.jpg" target="_blank">' +
                        '<img src="https://img.youtube.com/vi/' + theID + '/maxresdefault.jpg" /><figcaption>Download by: Right Click and Save Image as</figcaption></a>'
                    ).appendTo("#thumbnails");
                } else {
                    $('<div></div>').html('<div id="sorry">It seems like your "url" has an error</div>').appendTo("#thumbnails");
                }
            });
        });

        function showHide() {
            var div = document.getElementById("hidden_div");
            div.style.display = (div.style.display === 'none' || div.style.display === '') ? 'block' : 'none';
        }
    </script>
</body>
</html>