How Do I Kill A Enemy From Behind?

Hi y’all!

I’m trying to make it work so I can kill this enemy only from behind, like a stealth take down, but I get the error I posted under here when trying. I have a box collision behind the enemy that works as the “backstab-zone”.
I really dont understand why this doesn’t work, because I use basically the same thing to pick up an item, and there it works flawlessly. I have a box collision behind the enemy that works as the “backstab-zone”.

Can you guys help me out?