Hello, I've been recently getting to terms with Quester and so far so good but I've gotten to the point where I've made someone ask the Player to kill an NPC. The quest has been set up and once I toggle and start it, I'm told to go and kill the NPC as I wanted but when I go to kill him, it just comes up with 'No holder assigned'.

This is the current objective set for the NPCKILL.
Objectives:
[0] NPCKILL: Prisoner-4; AMT:1

I've also tried...
Objectives:
[0] NPCKILL: 22; AMT:1

since Prisoner-4's ID is 22 but neither of them work. Any ideas what's wrong?

Ah that must be it, thank you! The NPC literally just stands still so it's easily creatable again. Would it work if the prisoner didn't have the quester trait? Because I did remove the trait before hand and although I was hitting him, he wasn't taking any damage and only gave out flashes like hitting someone in creative mode.

Well, you need to sort that out yourself. It's either PVP protection of some sort or the NPC is not set to be vulnerable.

Does that prisoner have quester trait ? If so, I'm afraid you won't be able to kill that npc, because any interaction with it counts towards that trait.