A 2021 tax return could only be started using the TurboTax desktop download editions.
The TurboTax online editions were only for tax year 2022 and the online editions shut down after October 31, 2023 to make way for the 2023 online editions.
Go to this TurboTax website to purchase, download and install one of the 2021 desktop editions - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2021 tax return can only be printed and mailed, it cannot be e-filed using TurboTax.