What Kevin said is probably your easiest fix unless you are familiar with html/css coding.
The error basically means Sigil was looking for a tag to be closed properly and instead found an alpha character. The line number should be a good point to start looking for the offending tag.
I would also recommend using
Toxaris' Word Addon that does a GREAT job exporting a clean epub that you can easily upload into Sigil for final cleanup.
Cheers,