You downloaded the .tax2021 file from an Online account. Now you are trying to Transfer it to another Online account? Is that right? I'm not sure that is possible. Haven't hear of anyone doing it that way. It wouldn't matter if you are on Windows or Mac. You can try. I would think you would have to open the 2021 file in the 2021 Desktop program and save a new .tax2021 file from the actual installed program. Let me know if you get it to transfer or what you had to do.
Did you see this FAQ and try this? How to transfer into the Online version. You need to have the prior year .tax2021 file on your computer. Expand the link if you used the CD/download last year........
https://ttlc.intuit.com/turbotax-support/en-us/help-article/import-export-data-files/transfer-last-y...
If you already started an online return you will have to clear it and start over to transfer. How to clear and start over
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-clear-and-start-over-in-turbotax-onli...
GOOD LUCK
