You will be able to claim the full amount of the Child Tax Credit for your child on your 2021 tax return even if the other parent is receiving advance Child Tax Credit payments. The other parent should have unenrolled from receiving advance payments, but their decision will not affect your ability to claim the Child Tax Credit.
The other parent that is receiving the advance child tax credit payments will need to pay it back to the IRS if they do not qualify for repayment protection.
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-t...
