I am having the same issue with QB2021, and Mac OS BigSur.  I have  4 company files and I'm unable to back them up.  I've been using QB pro for 17 years, and have never had this issue.  I 'verified' data - it wasn't verified, so then I did the "rebuild", though I was still unable to back up the file, and I still can't back  it up.  I tried backing up to the external hard drive I use for my back-ups, and that isn't working either.  I don't know where to go from here.