The Online product is only for current year...2021. Each year's software is specific to that year. So if you need to file for 2021, you'll need to prepare your 2021 return using TurboTax Online.
To amend your 2020 return, you'll need to follow the instruction in the TurboTax FAQ I'm linking below. Please note that you must print and mail an amended return. Be sure to use Certified mail so that you can know that your return has been received by the IRS.
How do I amend my 2020 return?