I received a notice from the IRS that the amount due was more because of an error in the child tax credit on form 8812

[KeithA3]

How much is the child tax credit and why do I owe this money?

[Critter-3]

Sounds like the IRS is disagreeing on the amount of the CTC on line 28 of the form 1040 and this is usually because of the amount you got in advance that you entered in the TT program.  

If you filed jointly with your spouse, then half your advanced child tax credit payments would be reported to them. If the amount reported on Letter 6419 is exactly half of what you actually received, that is probably the reason why. 

If that is not the case, you first want to check on the IRS website to ensure Letter 6419 has the correct information for your account. If there is a discrepancy between what the website reports and what is on the letter, you should use the amount on the website. The website will also contain information for each payment so you can see exactly where the discrepancy is if there is one.

If the IRS shows a payment that you did not receive, you can request a payment trace from them by calling 800-919-9835 or mailing a Form 3911 for the missing payment. There are most likely to be problems with your payment if you closed the bank account that was receiving direct deposits or if you requested checks.

[xmasbaby0]

Did you receive advance child tax credit payments of $250 or $300 per child between July and December?   When the software asked if you received that money, did you enter the total amount received?   If you filed a joint return, then each spouse received letter 6419 showing half the amount received----and you were supposed to add it together when you entered the amount received.   Lots of people made mistakes and entered less than they actually received.   If you made that mistake the IRS recalculated your refund or tax due because you had an incorrect amount on line 28 of your tax return.

The total you could receive---including the advance payments---was $3600 for a child under age 6 and $3000 for a child age 6 to 17.   Line 28 was only for the difference between what you already received and the amount you could still get on your tax return.

Check your bank account for the payments sent to you between July and December.  Check your online accounts for EACH spouse if you filed a joint return.

https://www.irs.gov/payments/your-online-account

[SweetieJean]

See is this helps:

IRS revises 2021 Child Tax Credit and Advance Child Tax Credit frequently asked questions | Internal...