Online TurboTax is currently only for 2023 returns. To prepare a 2021 return with TurboTax, you would need to buy the 2021 desktop software and prepare it on your computer. It will have to be printed, signed, dated, and mailed. It's a good idea to mail by a method that has tracking, such as certified mail, priority mail, etc.
For 2021 desktop product:
https://turbotax.intuit.com/personal-taxes/past-years-products/2021/