We've created this Forum Thread with some workarounds and advice to assist you in publishing you ebook.
Little help with one Warning...
"Your EPUB file passes epubcheck-3.0.1, but we recommend you make a few corrections before proceeding."
"WARNING: [email protected]:uid content 'null' should conform to unique-identifier in content.opf: 'urn:uuid:e2e22071-7963-49a8-8d80-599e3e3ce47d' in OEBPS/toc.ncx"
This is from a Lulu generated EPUB that was downloaded, one or two small edits made in Sigil, then re-uploaded to Lulu . . .
Anyone get me started..? I'm almost there, I can smell it . . .