There are a couple of invalid attributes ("align" and "border") being used in line 80 of the "License & Copyright.xhtml" file inside your epub, Nelsea. You could try my and DaleDe's suggestions, above, and you might also find it useful to run the epub through
FlightCrew, an Epub validator created by the guy who created Sigil - it gives more easily understandable error messages with more information.
Edit to add: If you're not sure what change(s) to make to that HTML file to fix the error, copy and paste the problem line from that file here, and someone will help you identify what needs to be changed.
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