"What do I need to do???"
Call your congressman. That's the law. I've never understood why they think a 17 year old cost less to support than a 16 year old!
It gets worse: you said you child turned 18 in 2016. This means your were also not entitled to the Child Tax Credit (CTC) in 2015. If you erroneously claimed it, you will need to file an amended return for 2015.
