Under the rules for the child tax credit on a 2020 tax return, you do not get the CTC for a child who turned 17 before the end of 2020 so that is why you could not get the credit for your 2020 return.
Under the rules for your 2021 tax return, you only get the CTC if your child is UNDER the age of 18 at the end of 2021.

Who Else Wants Advance Child Tax Credit 2021 Without Stress?
- You will still be able to claim your child as a dependent for 2021, but you will get the $500 credit for other dependents, not the child tax credit.
- If your child becomes a full-time student going forward you can continue to claim the child as a qualifying child dependent until the tax year when the child turns 24.
2. You can claim a child, relative, friend, or fiancé (etc. (https://winandoffice.com) ) as a dependent on your 2020 taxes as long as they’re related to you.
When you add someone as a dependent, you’ll be asked a series of questions to make sure you can claim them.
There may be other tax benefits you can get when you claim a dependent. Visit our office and talk to our Generational wealth specialist for more.