The letter from the IRS probably is stating that you indicated on our 2020 federal tax return that you did NOT receive a stimulus payment and therefor the amount that you did not receive was entered on the Form 1040 Line 30 as a Recovery Rebate Credit.
However, the IRS removed the credit from your tax return because the IRS shows that the stimulus payment was sent to you. By removing the credit this reduces your federal tax refund or increases the taxes owed.
Go to the IRS website for your tax account to see what the IRS has sent to you for stimulus payments - https://www.irs.gov/payments/view-your-tax-account
If you never received the stimulus payment shown on your account then have the IRS put a trace on the payment. Go to this IRS website for how to start a trace - https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-f-p...
Line 30 is the Recovery Rebate Credit. You only should have an amount if you didn't get some of the first 2 stimulus payments or qualify for more. Did they take it off your refund or ask you to pay it back?
A common adjustment is if you claimed the Recovery Rebate Credit on 1040 line 30 when you already got the 1st or 2nd Stimulus payments or got the second one after you filed or reported the wrong amounts. Do you have an amount on line 30? And you had to report the right amount in each box. If you put the total in 1 box it would give you the other amount again. The IRS knows they already sent it to you. Line 30 is only if you qualify more.
The first round EIP1 was 1,200 per person or 2,400 for a couple and 500 for each dependent under 17. The second round EIP2 was 600 for each person listed and dependents under 17. Did you get the full amount for each of those?
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.
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.