The IRS cross checks every Form 1040 with an amount on line 30 for the recovery rebate credit. If the IRS thinks you already got the EIP payment, you cannot get it again as a tax refund. If you claimed a recovery rebate credit for a payment already sent to you, they recalculated your refund.
Check your IRS account and see if the IRS sent your EIP payment. If they did, and you really did not receive it then they can trace it.
If you were issued a payment and have not received it, see How do I request a payment trace to track my first or second Economic Impact Payments?
The first stimulus payment, EIP1, was $1,200 for each taxpayer plus $500 for each eligible dependent and issued in early 2020. The second stimulus payment, EIP2, was $600 for each taxpayer plus $600 for each eligible dependent and issued 12/2020 thru 02/2021.
If you received both payments you should have indicated that on your 2020 federal tax return and No credit would have been entered on Form 1040 Line 30.
If you did Not receive one or both of these payments you should have indicated that on your 2020 federal tax return and the Recovery Rebate Credit would have been entered on the Form 1040 Line 30.
The IRS notice you received means the IRS has records showing that the IRS sent you the stimulus payment and has removed the Credit from your tax return.
Look at your tax account on the IRS website for the stimulus payments sent to you by the IRS - https://www.irs.gov/payments/view-your-tax-account
If you never received the stimulus payment then you need to start a trace on the payment with the IRS. 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...