I bought an .azw3 book book and loaded it to Calibre. I converted it to .epub and loaded it to a bunch of e-readers on my phone and pad. But it failed to be processed with Google books. When I tested it with
https://www.ebookit.com/tools/bp/Bo/...epub-validator, I found lots of errors. Lots. I don't have the expertise to fix them in Sigil. How do I give feedback to whomever should know what the problem is?