It doesn't fail.
Sigil's role is to edit epub ebooks not KDP. There are many tags not supported in KDP and they like their input formatted in specific ways.
Have you read and followed the Kindle Publishing Guidelines?
Perhaps an ebook converter and editor like calibre will do what you want?
If you decide to stick with Sigil then allowing Sigil to Mend your code might be a good starting point. Then using the epubcheck plugin in Sigil to validate your epub. Fix any validation errors.
Once you have a valid epub2 or epub3, you should be able to use calibre, or kindlegen or KindlePreviewer to convert your valid epub to their format.
Last edited by KevinH; 12-25-2020 at 08:36 PM.
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