No, you do not have to amend the 2021 tax return if you file the 2020 tax return after the 2021 tax return is filed, in most cases. Only if you have carryovers from the 2020 tax return, which is not usual, would you need to amend the 2021 return for the carryovers.
To complete and file a 2020 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2020 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2020 tax return can only be printed and mailed, it cannot be e-filed.
