The problem is really with epubcheck, which iTunes uses under the hood. epubcheck generates tons of useless errors/warnings for things that are not actually problems. If you paste some of those error messages someone can probably tell you how to fix them. Note that calibre guarantees that the epub it produces will satisfy epubcheck only if you give it input HTML/CSS that satisfies epubcheck.
https://manual.calibre-ebook.com/faq...e-is-not-valid