It actually ends in .tax2020. You need to have the file ending in .tax2020 to transfer in to 2021.
Could you have transferred from the wrong file? See if you have another .tax2020 file on your computer, probably in Documents.
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. Then save it again with a name and place you can find. Go to File - Save As or Duplicate.
