Chosen Solution
I did a screen replacement on a Note 5, and everything works but the S Pen. I tried another screen, S Pen still doesn’t work. Tried a different S Pen, and that one doesn’t work as well. I’m stumped.
Did the new screen have a stylus pen flex cable? Is the cable clicked in properly? If they are then try hard rebooting. If not then the stylus flex could be faulty.