It sounds like you are trying to open the old tax return with the CD/Download TurboTax software, not TurboTax Online. Did you originally file it with the 2020 CD/Download software or with TurboTax Online?
If you are trying to open a .tax2020 file in the 2020 CD/Download software, and the file contains a state tax return in addition to the federal, you must install the state software before you can open the file, even if you only want to look at the federal return.
