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.
You can complete and file a 2021 tax return without having a 2020 tax return completed. When e-filing the 2021 tax return and asked for the 2020 AGI just indicate that you did not file a 2020 tax return.