I noticed that  <title><?php wp_title( '|', true, 'right' ); ?></title>  in the header had been modified, so I changed it back to  <title><?php wp_title( '|', true, 'right' ); ?></title>   Please don't modify it again, this code is responsible for the posts and pages to have different page titles, each page/post with its own page title, that is very important to search engine optimization.