Sorry you can't use the Online version. Online is only for current year 2022 returns.
If you need to start 2021 you can buy the Desktop download here,
TurboTax 2021 Prior Year Tax Prep - File Past Years' Taxes
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 2021 1040 return
https://www.irs.gov/pub/irs-prior/f1040--2021.pdf
or if you want bigger type use 1040SR for Seniors,
https://www.irs.gov/pub/irs-prior/f1040s--2021.pdf
2021 1040 Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2021.pdf
2021 EIC and Tax Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2021.pdf
Schedules 1-3
https://www.irs.gov/pub/irs-prior/f1040s1--2021.pdf
https://www.irs.gov/pub/irs-prior/f1040s2--2021.pdf
https://www.irs.gov/pub/irs-prior/f1040s3--2021.pdf
