iBooks for Mac can read the epub, but Google Play Books rejects it: "This file cannot be processed." The epubcheck utility reports more errors than I know what

iBooks for Mac can read the epub, but Google Play Books rejects it: "This file cannot be processed." The epubcheck utility reports more errors than I know what to do with. For example: Error while parsing file 'value of attribute "lang" is invalid; must be an RFC 3066 language identifier'. Error while parsing file 'value of attribute "id" is invalid; must be an XML name without colons'. Error while parsing file 'element "figure" not allowed anywhere; expected the element end-tag, text or element "a", ..., "ul" or "var" (with xmlns:ns="http://www.w3.org/2000/svg")' http://www.w3.org/2000/svg")'. Is this likely a problem in the sources, GitBook, or something else about how the submitter built it?