Ugh! This sucks so bad!!
Many of us, included me, found that after we had upgraded to Big Sur, if we uninstalled our download of TT from the fall, and removed all associated files from our hard drives, and then downloaded TT20 again (it was by this time a different version that we'd downloaded in the fall), and installed it, it worked fine.
The instructions for deleting the associated files are earlier in this thread. I will just say that after following those instructions, it was a bit of a problem to import my TT19 files into TT20, but I figured it out eventually.
Good luck! All is not lost!!
