Error message when trying to deliver

I am trying to deliver my book (created in iBooks Author) to the iBooks store. I get an initial message that my package has been validated but at the end of the process I get the following message:

ERROR ITMS-9000: "OPS/ibooks.ncx(5): element "content" not allowed yet: missing required element "navLabel"" at Book(CMZltmspBook Package)

What does this mean and how can I fix it?

Is there a glossary somewhere that explains these error messages?

Should I email Apple support about this issue?