If you sent your payment with that 2020 return, check to see if it was cashed. If it was cashed, then the IRS also received your return for 2020 (you sent them together) and you will not need to send another copy of your 2020 return. The IRS is just way behind entering returns for 2020, likely due to Covid.
I have also seen letters issued by the IRS which has asked people to send a copy of the 2020 return, but those were letters generated automatically because, as noted above, the IRS is still way behind entering 2020 returns. The IRS has now stopped those automatic letters, and is asking taxpayers NOT to (re)send their 2020 returns because that will only cause more confusion for them.
