I'm trying to share my 2020 File with my CPA. She was able to open 2018 and 2019, so she knows what she is doing, and knows she has the 2020 desktop app (as do I). So why can't she open it? I sent her to .tax file via email and uploaded into her file service just like I did with the 2018 and 2019 files.
"Can't open" is not nearly enough detail to be able to diagnose a problem.
Is she using TurboTax or professional software that can open TurboTax files? If TurboTax, what edition (Deluxe, Premier, etc.)? On a PC or a Mac? If she's using professional software, she should contact support for her software.
What happens when she tries to open the file? If there's an error message, what is the complete message?
You said you have the "desktop app." Do you mean the CD/Download TurboTax software? If so, what edition? Are you using it on a PC or a Mac?
Did you put a password on your tax return?
Does your .tax2020 file include a state tax return? Does the CPA have the software installed for the same state?
You said you sent the file "via email and uploaded into her file service." That's two different ways of sending it. Did you send an email directly to the CPA with the file as an attachment, or did you upload the file somewhere? What is her "file service"? If you uploaded it somewhere and she is retrieving it, the file service might have done something to the file (even if it didn't do it in past years).