Yes, it looks like "Stun" is incorrect.
I just tested this and it seems to be working. Maybe you need to define the Force parameter as well.
IF CharacterEnteredTrigger(_Player, TRIGGER_HUS_ChatTrigger)
THEN CharacterApplyStatus(_Player, "STUNNED", 6.0, 15);
EDIT: I just tested without the Force parameter and it is still working.