Are you providing support for this child?  Are they related to you?  

Here are two articles that will help you decide if this child is your dependent:  7 Requirements for the Child Tax Credit   What is the Child Tax Credit?

One reason is they have to be UNDER 17 on Dec 31, 2019.  So he's too old.

Yes, ma’am he’s my young brother and has been living with me the past three years I have custody of him from the state this is the first time this has happened 

Is this a new rule regarding the child tax break because I’ve meet with people who claimed 18 year olds and still got the break 

He should qualify for the child tax credit if he qualified in the previous year and nothing changed but his age. However, the child tax credit is only a maximum of $500 starting in the year the child turns 17. So, you may have the credit on your return, it may just be much smaller than you are used to. 

No it's always been that way for the Child Tax Credit.   See,

https://ttlc.intuit.com/community/credits-and-deductions/help/what-is-the-child-tax-credit/00/25961 

IRS publication 972 for Child Tax Credit,  https://www.irs.gov/pub/irs-pdf/p972.pdf 

Look at it this way: when they were born (even if on Dec 31), you got the full $1000 for the whole first year, So, you got 17 years of the child tax credit.   

@ThomasM125   When they turn 17 they don't get the Child Tax Credit anymore.  Now they get the Credit for  Other Dependents.  

That’s what I thought but it keeps telling me he doesn’t qualify @ThomasM125 

Your child tax credit cannot be more than your taxable income, that could be a reason you aren't getting the credit. 

Otherwise, you should go back through the questions regarding your child to see if he is qualifying as your dependent.