You can no longer use TurboTax Online to file for 2020. TurboTax Online is only for the current tax year, which is now 2021. To file for 2020 you have to use the CD/Download TurboTax software for 2020. You can purchase a download at the following link. There is no free filing for past years.
You might get a better price from a reputable online seller that still has TurboTax for 2020 in stock.
Tax returns for past years cannot be e-filed. You will have to print your 2020 tax return, sign it, and file it by mail.
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.pd
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.