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If you're getting an error message, then most likely something is wrong with your book spine settings. In the BookBildr Editor, click on “Spine” and make sure that the background color and text color are using a HEX code (not rgb, and not words like “red” – that would break the settings and result in an error). Once you’ve fixed the spine settings, save your project and try checking out again. If the problem persists, contact us via the contact form on this website.