[KevinH]

Your script tag contains xhtml reserve characters such as '<', '&', or '>' that confuses the parser because it is not protected by a CDATA tag pair:

'<![CDATA[' and ']]>'

So edit that file before import and add that cdata tag pair inside that script tag or
alternatively you could edit that html file and extract that script and paste that script into its own file and just properly link to it.

Sigil will not allow you to import it until it is fixed mainly for security reasons.

If this is something you run into often, then the best solution is to use a Sigil plugin to add selected files after removing their broken script tags.

Sorry.