Only tax year 2021 can be filed using the TurboTax Free online edition.
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.
visit the IRS at https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free
to find truly free filing. Note that some include free state as well; that's important.
Otherwise, If you have to file a state return, and don't want to do that yourself, you would need software that you pay for.
If you have a simple return and want to file for free you can fill out the forms by hand. Here are some basic forms.....
Here is the 2020 1040 return
or if you want bigger type use 1040SR for Seniors,
2020 1040 Instructions
2020 EIC and Tax Tables
Don’t forget your state. And you will have to print and mail your returns. So be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
Then when you file for 2021 you need to enter 0 for the 2020 AGI since you are filing 2020 late.