can i get child tax credit for my child who is 18 and going high school

No.  The child must be under the age of 17 to be eligible for the child tax credit.  You can receive the Other Dependent credit of $500 when claiming them as a dependent.

No, you cannot claim the Child Tax Credit for your child who is 18 and still in high school. The Child Tax Credit is only available for qualifying children who are under the age of 17 at the end of the tax year

However, your 18-year-old child may still qualify as a dependent and for other credits, such as the Earned Income Credit (EIC) if they meet certain criteria

For more detailed information, you can visit the TurboTax Child Tax Credit Guide

and the IRS Child Tax Credit page