epub having problems showing up in validator

I'm getting a similar error message: "(RSC-012): /OEBPS/toc.xhtml: Fragment identifier is not defined."

I loaded my ebook to Amazon with no problems, but when I try to upload to Ingram Spark I get the above error message and am not able to proceed. After several trys, I stripped my book down to 2 pages - one page worked if I switched to Georgie type (which is a TT type). But if I added a second page with the same sentence, I get the above error message.

So far, I don't know what to do!

Validation is a necessary step for the legacy methods of creating an EPUB, where the individual components are created separately and then combined to create the EPUB file. There are many, many mistakes that can be made in the files, naming, structure, metadata etc. when "building" an EPUB this way.

When exporting from a modern tool like InDesign, not so much. Validators are a seldom-needed diagnostic tool at best, because much of what they check is done flawlessly by InDesign, with no need to "validate" it.

If your doc "looks and works great on Calibre" — and kudos for choosing a standard, "vanilla" reader to proof your work! — then I'd say you're done. Nits found by a validator are largely meaningless. (If you're not using EPUBcheck, which is the equivalent standard validator not dressed up with extra help and a paywall, you might try it for an alternative view.) But the EPUB world is so variable that expending much effort to resolve these minor INFO issues would be lost in other incompatibilities with the wide array of EPUB readers that will be used.

But it looks as if you do have a few genuine problems that can and should be resolved in InDesign (that is, not by exporting a broken file and then trying to patch it up, old school style, in something like Sigil). But it looks as if you are exporting to (1) fixed layout EPUB (FXL) and (2) are embedding fonts... which are both obsolete practices. FXL is almost impossible to fix; you either export it perfectly or you don't. And fonts are usually encrypted for use with that document and no other, and many things can go wrong with every reader on every platform decrypting them properly. (Your doc probably works because you still have those fonts on your system, and Calibre is drawing them from there. Any user who did not have those fonts would see a substitution, or garbled text.)

I infer that you are also including (3) interactive elements. Those, too, can be problematic in the variety of readers out there, including the "plain standard" ones like Calibre and Thorium. My recommendation is to use only reflowable EPUB format; to not embed (or even spec) fonts at all; and to avoid any interactive elements beyond simple cross-doc links. If that won't work for you/your project, all fixes will have to be done within InDesign to get the fixed layout and/or interactive elements to work correctly. There are many subtle aspects to those, especially the latter. Other experts here have walked users through the interactive issues, though.