Are you on Windows or Mac? It should transfer. Are there any other files listed? Did you try all the .tax2020 files?
Here's an idea. Do you still have 2020 installed? Try opening 2020 program and see if you can get your 2020 return to show up in it. Go to FILE - OPEN. And see if that is the right one.
Then in the 2020 program do a File-Save AS and give it a easy name you can find or search for. And note where it saves it or select where to save it so you can find it again. The file will end in .tax2020. Then close the 2020 program. Then start a new 2021 return and transfer from the 2020 file.
If you don't have the main .tax file you might have one that starts with a Tilde sign ~ like "~your name.tax2020". That file is if your computer crashes or your real file gets lost or deleted or corrupt so you can restore it.
