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 using TurboTax.
Sorry, Online is only for 2023 now. Online for 2022 and prior is closed now so you can not start a new 2020 or prior return or finish one. You have to do each year separately
To do a prior year return you have to buy the Desktop program here,
https://turbotax.intuit.com/personal-taxes/past-years-products
You will need a full Windows or Mac to install it on.
If you want to file for free you can fill out the forms by hand. Here are some basic forms…..
Here is the IRS 2020 Form 1040
https://www.irs.gov/pub/irs-prior/f1040--2020.pdf
or if you want bigger type use 1040SR for Seniors,
https://www.irs.gov/pub/irs-prior/f1040s--2020.pdf
And 2020 Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2020.pdf
2020 Tax and EIC Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2020.pdf
Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2020.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2020.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2020.pdf
You have to print and mail prior year returns. When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return. Don’t forget state.
Important! Mail Federal and State in separate envelopes and mail each year in separate envelopes.