The "laundry list" of possible errors on the CP11, the only one that applies for the CTC and/or RRC is the last one being a "math" error. This had to do with the amount you listed for the advance CTC payments and/or stimulus payments. The IRS disagreed with what you entered based of their records of the payments sent to you and have adjusted your return accordingly.
If you claimed a missing stimulus payment on your return but the IRS took it off you have to ask the IRS. They think they already sent it to you. Maybe it went to an account you don't remember. Or you got a check or a card. It was probably easy to miss the debit card in the mail and think it was junk mail. They have to put a trace on it.
Double check your bank statements and Check your IRS account to see when and what they sent you (check both spouse's accounts if married)
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...Your Online Account | Internal Revenue Service
