Probably because you did not indicate that he lived with you for the entire year. On the screen for entering how long he lived in your home was information on how to report if the child was born in 2019.
Go back to the My Info section and edit the dependent section to correct the mistake.
Sadly many people fail to read the screen that tells you exactly what to do ... pay attention to all the words on the screen ... take your time and read them all on every screen ...
And--have you entered your income earned from working yet? Unless you have entered your W-2, you are not going to see any child-related credits. Employers have until January 31 to issue your W-2.