The IRS Free File version of TurboTax ended with last year's return (Tax Year 2020). Prior year tax returns can no longer be amended for e-filing. However, you should be able to login at turbotax.com, see last year's return (possibly unless you've already started this year's Free Edition at turbotax.com which is different from IRS Free File), and download that return as a .tax2020 file along with a limited free copy of TurboTax CD/Download 2020 that should allow you to prepare an amended paper return for 2020. Same back to either 2017 or 2018.
If you meant a Tax Year 2021 return filed with Free Edition, amended 2021 returns won't be available till later in the tax season.
