You cannot use Online TurboTax to prepare a 2020 return. Online TurboTax is currently only for 2021 returns. To prepare a 2020 return with TurboTax you have to use the 2020 desktop version (CD/download).
You can purchase 2020 desktop software directly from TurboTax as a download at this page and install it on your computer:
https://turbotax.intuit.com/personal-taxes/past-years-products/
Or you may be able to find the 2020 software cheaper at a reputable authorized retailer like Amazon, but even there be sure it is a legitimate authorized product from Intuit sold directly by Amazon itself, since there's no way to be sure all third-party sellers on Amazon are legitimate.
Be aware there are "too-good-to-be true" Internet scam sites selling fake or pirated software.
Minimum system requirements for Windows version:
https://ttlc.intuit.com/questions/1901571-minimum-system-requirements-for-turbotax-windows-software
Minimum system requirements for Mac version:
https://ttlc.intuit.com/questions/1899173-minimum-system-requirements-for-turbotax-mac-software
The 2020 return cannot be efiled. It will need to be printed, signed, dated, and mailed. It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc.
If you have both Federal and state returns, you need to mail each one separately to its respective tax agency.
