I received a letter from the IRS that my recovery rebate amount is wrong

Early in the 2020 tax season some returns had the stimulus payment added and it was up to the user to remove those payments if they had already received their payments.

If a return was filed with a recovery rebate added to the refund, the IRS checked by hand to be sure that the stimulus had not been paid to the person requesting it.  If that person already received the stimulus, the IRS sends a letter to inform that the payment is not owed.  

Check to be sure you received both your first and second stimulus payment.  If you did not receive them and the IRS says that you did, they will have to trace what happened to the payments.

The credit was computed by the TT program based on the information you entered in the program ... it asked how much you got in advance on the first 2 stimulus checks and if you did not get the max allowed then the missing amount becomes a credit on the 1040 line 30.   

If you claimed a missing stimulus payment on your return but the IRS took it off you have to ask the IRS. They think they already sent it to you. Maybe it went to an account you don't remember. Or you got a check or a card. It was probably easy to miss the debit card in the mail and think it was junk mail. They have to put a trace on it.

How to put a trace on a missing 1st or 2nd Stimulus payment.  IRS for Payment Issued but Lost, Stolen, Destroyed or Not Received

See question F3 here, you can expand it to see how to put a trace on it.

https://www.irs.gov/newsroom/2020-recovery-rebate-credit-topic-f-finding-the-first-and-second-econom...