Well the letter the IRS sent doesn't sound like the Stimulus payment adjustment but whatever......
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. Did you get the 2nd round after you filed your return? 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...
Line 30 is only if you didn't get one or both of the first 2 Stimulus payments or if you qualified for more, like if you added a dependent. Did you not get the full amount for the first two EIP payments? Maybe you got the second one after you filed?
