why is my s pen not working and cannot write on the screen off ?

my s pen is working for all features except that i cannot write in the screen while it is off ( black ) , please advise

You can't do anything on the screen when it's off. That's how the phone works.

You know, I really wish this site would force people to list their phone model!

If this is a Note 5, look in your S-Pen settings to make sure you have screen off memo enabled. Heaven knows if this will help because I have no clue what kind of phone you have.