What computer are you trying to download it too? Why do you want the .tax2020 data file? Do you have the 2020 Desktop program installed to open it? You need the file ending in .tax2020. I use Chrome on Windows and don't have trouble.
Here's how to move from Online to the Desktop program.
First download the .tax file from the Online version,
https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-tax-data-...
Then see, How to Switch to Desktop,
https://ttlc.intuit.com/community/choosing-a-product/help/how-do-i-switch-from-turbotax-online-to-th...
After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. So you first might need to start a fake return to be able to download the state program (go to FILE - New)
Then you just OPEN the .tax2021 file, go up to File-Open. You don't import or transfer it.
