@vbugata - normally the error is a mismatch between what you wrote down that you received for the Advance Tax Credit and what the IRS records indicate they sent you.
1) Check your bank account records from July - Dec, 2021. How much was deposited each month? What is the sum for the 6 months.
2) What is on line 14F of Form 8812? You were asked during the interview process how much you received for the Advance and your answer is reflected on this line.
these amounts needed to match, If they don't, this is why the IRS is asking for money
