That is one of the generic reasons why they adjusted the Recovery Rebate Credit on your return. Do you have an amount on 1040 line 30? See this IRS article
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.
When I filed I filed for missing stimulus checks I never received them so I winded up getting them when I got my taxes they want me to pay back the stimulus checks because they said that I have a dependent in the information for that dependent is wrong I never filed a dependent I don't have any children and they're trying to say that they're going to garnish my wages ages for the stimulus that they paid me for
Well the IRS thinks they already sent them to you. See Question F3 for how to put a trace on a missing payment
First stop focusing on the first possible reason on the generic list they sent on that notice ... it is only the last one ... the stimulus credit you took on your return that they are focusing on ... look on the form 1040 line 30.
Now did you say you got the missing stimulus money AFTER you filed with the credit on the return ??? If so the IRS is saying you cannot have the credit at all ... you don't get the stimulus payment AND the credit. So they removed the credit from your return and now you don't have a refund you have a balance due. Review your return ... remove the credit on line 30 and recalculate the rest of the return ... is the IRS correct ?