A common adjustment this year is if you claimed the Recovery rebate credit on 1040 line 30 when you already got the Stimulus payment or got the second one after you filed or reported the wrong amounts or entered the wrong amounts in the 2 boxes. If you entered the total you got for both rounds in the same box it would think you didn't get the other one.The IRS knows how much they already sent you. You might have tried to claim it again on line 30.
When you get the promised notice you will know what they changed. Was there an amount on the form 1040 line 30? Was that amount your entire tax refund ? If so most likely the IRS declined the credit ...
The letter from the IRS probably is stating that you indicated on our 2020 federal tax return that you didNOTreceive a stimulus payment and therefor the amount that you didnotreceive 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.