Read the notice carefully ... are they correct ? Did you enter the wrong amount for the advance CTC payment you got last year ? Is the amount in question listed on the form 1040 line 28?
First---check your own bank account for the amounts you received for advance child tax credit payments between July and December. Add them up---do not rely on your memory. Then look at the letter 6419 you received from the IRS that showed the amount sent to you. If you filed a joint return, then look at the letters for both spouses.
How did you answer the questions about the CTC on your tax return? Did you enter the correct total received? What was on line 28 of your tax return? Was that amount the correct "second half" of the child tax credit you were eligible to receive? Or was the amount on line 28 overstated because you entered an incorrect amount for the advance payments?
Does the IRS show that you received payments that you did NOT receive? Check your online IRS account. Check the accounts for BOTH spouses.
https://www.irs.gov/payments/your-online-account
If you need to trace a payment that the IRS says has been issued to you, mail or fax a completed IRS Form 3911, Taxpayer Statement Regarding Refund.
1) look at line 14f of Form 8812 - that is what you told turbo tax you received as the advance.
2) Now, go look at you bank statements from July - Dec, 2021 and review what you actually received. DO NOT DO THIS FROM MEMORY. Look at all 6 bank statements,
if these two items do not match, that should be the amount the IRS adjusted your return by,
if that doesn't solve the problem, post back and I should be able to figure it out by asking a few more questions