You can file 2021 first.
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 file for 2021 you need to enter 0 for the 2020 AGI since you are filing late.
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.
You cannot do so online. Rather, you must purchase and install the 2020 software.
TurboTax® 2020 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)
Or, you can pull the forms and instructions from the IRS and state tax websites, and do them manually. In any case, you must file by mail.