Assuming you are talking about the Apple
Books app. It expects things to be 'epubcheck' compliant.
There's an EpubCheck plugin:
https://www.mobileread.com/forums/sh...d.php?t=282067
It requires Java runtime installation on your computer.
There's also a Quality Check plugin, which might find some of the issues
Books is complaining about (and does not require Java):
https://www.mobileread.com/forums/sh...d.php?t=125428