Why can’t I claim a student still in high school that turned 18 in July but still had a year to finish. I’m his guardian been raising him for years

You might be answering something incorrectly in MY INFO.   If he is older than 16 you cannot get the child tax credit, but you can get the $500 credit for other dependents.

Make sure you have entered your child as a dependent in My Info, and that you have entered the child's Social Security number.    Careful— do not say that your child’s SSN is not valid for employment.  If your child was born in 2024 make sure you said he lived with you the whole year.  There is an oddly worded question that asks if the child paid over half their own support.  Say NO to that question.

If you are his legally appointed guardian, for relationship, you answer that he is your  child (which includes a foster child).