Sounds like Access is getting confused about the file's internal version or read/write status, which usually points to minor corruption or a problem with how the file is being opened. The "unconverted" message means Access thinks the database was created in an older version and has not been fully upgraded. That can happen even if it is already .accdb, especially if it has been compacted or copied a few times.

Make sure the file is really an .accdb (not just renamed from .mdb), and check that it is not marked read-only. Try Compact and Repair, and if that does not help, create a new blank database and import everything from the old one. If it is split, make sure the back-end is not locked by another user or stored on a flaky network share.

When in doubt, run down my Database Troubleshooter.

Thank you Richard

I put this together from online classes at the local Community college, some info on the internet, and from various tech-help videos from you. But now I am seeing that I need to do some rebuilding by following your training. there are some forms, Queries that I can make better by working thru your training.
I do enjoy your "Captain's Log and all the tech help videos.