You need to use a 2020 desktop version of TurboTax (link below).

See TurboTax® 2020 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)

You cannot use an online version to prepare a 2020 return.

Sorry Online is only for 2021 now. It's too late to start a 2020 online return or use the mobile app, either on Turbo Tax or the IRS.

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 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
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

2020 1040 Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2020.pdf

2020 EIC and Tax Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2020.pdf

Schedules 1-3
https://www.irs.gov/pub/irs-prior/f1040s1--2020.pdf
https://www.irs.gov/pub/irs-prior/f1040s2--2020.pdf
https://www.irs.gov/pub/irs-prior/f1040s3--2020.pdf

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 do file 2021 enter 0 for the 2020 AGI since you are filing 2020 late.