First, a child does not claim the child tax credit (CTC) on their own tax return.
The parent claims the child for the child tax credit, if the child qualifies as a dependent. Any child filing for a recovery rebate credit (stimulus) would not have qualified as a dependent..
For tax year 2020, 17 year olds do not qualify for the CTC .
It is not clear exactly what your teenager did on a tax return. At the age of 17, that child can be claimed as a dependent on a parent's tax return---so if they said they could not be claimed that was a mistake. Dependents are not eligible to receive stimulus checks---so if your teenager said they could not be claimed as a dependent and did receive stimulus money---that was actually quite incorrect.
Your teenager could be claimed as a dependent on YOUR tax return and YOU could receive the 3rd stimulus payment for the dependent. You could not get the 1st or 2nd EIP payment for a child who was older than 16 at the end of 2020, but the law changed to allow you to get the 3rd payment for any dependent claimed on your return.
As for the child tax credit--- under 2020 tax law, the parent cannot get the CTC for a child who was older than 16 at the end of 2020. And the child herself could NEVER receive the child tax credit.
It sounds like your child needs to amend her tax return---and perhaps pay back some stimulus money. And if you filed without claiming your teenager, you can also amend in order to claim her for the $500 credit for other dependents and for the 3rd stimulus check.