Did you click on the DETAILS button to find out what they changed ???
Code 9021 is for a "math error" so did you have an amount on the form 1040 line 30 ? If so what that same amount your total refund ? If so the IRS disagrees with the credit on that line.
If the IRS says the stimulus was sent but you did not get it then you will have to have a trace put on the missing payment:
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
