You are correct that you need to amend your 2020 return before you can even start your 2021 tax return. It's most important that when you start your 2021 tax return, that you ensure you are importing from the amended 2020 return, and not the originally filed 2020 tax return.
Before you amend, get your AGI of the originally filed 2020 return, as you will need that original AGI in order to e-file your 2021 tax return. If your amending changes your 2020 AGI, then the AGI that the IRS does not have on file for 2020 will be imported into your 2021 tax return. But you will have the opportunity to change it to the original 2020 AGI so you can successfully e-file the 2021 tax return.
How you ensure you import the amended 2020 return depends on weather you used the online version of desktop version to prepare your 2020 tax return, and which you are using for your 2021 return as well.
