Hi. As the help page says, the option to convert local notebooks comes up automatically during the upgrade process, so maybe your separate activation caused some confusion, or there was a problem with one or more of the notes in your local notebook which crashed the feature.
I'd suggest you sign out of Evernote and power your system off, then back on, then run the check for local notebooks again. Your original EXB file should still be on your system containing the rest of your files. Hopefully the system will recognise them and start the import again. It's possible that the existing imported notes will be duplicated - it might be an idea to move them into a temprary notebook before you retry this exercise just in case.
If this doesn't resolve your issues, I'm afraid it's a Support problem - they're rather busy now, so responses might be a little slow, but definitely contact them about this.