After you file

The first 2 stimulus checks were being reconciled on the 2020 return and the third one will be reconciled on the 2021 return.  If you had gotten them then you had to enter them both ... the screen told you just that.  Failure to enter the correct amount you got in advance will be corrected by the IRS and the return is recalculated without that credit which means you could either get less of a refund or actually owe so they bill you.