The only reason you would owe money back for the advanced child tax credit would be if you were paid more than you were entitled to.
One scenario that would cause you to pay back advanced child tax credit would be that you share claiming your children with someone where you claim one year and they claim the next year. If this is the case, this is why you owe back the advanced child tax credit. If you claimed the children on your 2020 tax return, then you received the advanced child tax credit. If you do not claim the children on your 2021 taxes, then you were not entitled to the advanced child tax credit since this credit is actual for the 2021 tax return.
The only other scenario I can think of would be if you received an advanced child tax credit for a child that you should not have because of age.
