The TurboTax online editions are only for tax year 2021.
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.
If you MUST efile the 2020 return NOW you will have to find a local tax pro who can still efile a 2020 return at this time.
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
Then when you do file 2021 enter 0 for the 2020 AGI since you are filing 2020 late.
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.