Sorry--- you do not get the child tax credit for a child who will be older than 17 at the end of 2021. She will be 18 before the year ends. You will still be able to claim her as a dependent on your 2021 tax return and you will get the $500 credit for other dependents.
If she is your dependent she would qualify for the Other Dependent tax credit of $500 which should have been entered on your 2020 tax return, Form 1040 Line 19. She is not eligible for the Child Tax Credit in 2020 or in 2021 due to her age.