It's not a scam. Many people are getting those CP80 letters. It seems that the IRS just sent out a big batch of them. The problem is that the IRS is severely backlogged in processing 2020 tax returns. They deposited your payment and recorded it when you sent it, but they are so far behind that they haven't processed your 2020 tax return yet. So their computerized records make it look like you made a payment and didn't file a tax return.
Probably your best course of action is to just do what the CP80 notice says and send a complete new signed copy of your 2020 tax return to the address in the notice. The IRS will eventually straighten it out.
