Why does my canteen says no prisoners assigned?

I had the same problem, and soon found a reddit thread that explained why this was happening:

To get to the canteen prisoners have to go outside where there is no blocked off area preventing them from escaping. The game prevents any prisoners from accessing this area because they could escape. Try fencing the outside of your prison off.

I also had this problem.

I forgot that I'd set the entire outside area to "Staff Only" in the Deployment tab while I was building the new prison area, and ended up starving a bunch of my prisoners for a few days until I figured out why they weren't going to eat! Whoops!