Смотря какая строка имеется ввиду. Скорее всего, не указан тип параметра akActionRef.
Вот накидал рабочий скрипт:
Scriptname SRNewAutoLights extends ObjectReference
Int _actrosCountInTriggerZone = 0
Int Property actrosCountInTriggerZone
Function Set(Int newValue)
If newValue < 0
newValue = 0
EndIf
If newValue != _actrosCountInTriggerZone
If newValue == 0 && _actrosCountInTriggerZone != 0
TriggerActionStop()
ElseIf newValue != 0 && _actrosCountInTriggerZone == 0
TriggerActionStart()
EndIf
_actrosCountInTriggerZone = newValue
EndIf
EndFunction
Int Function Get()
Return _actrosCountInTriggerZone
EndFunction
EndProperty
Event OnTriggerEnter(ObjectReference akActionRef)
If akActionRef As Actor != None
actrosCountInTriggerZone = actrosCountInTriggerZone + 1
EndIf
EndEvent
Event OnTriggerLeave(ObjectReference akActionRef)
If akActionRef As Actor != None
actrosCountInTriggerZone = actrosCountInTriggerZone - 1
EndIf
EndEvent
Function TriggerActionStart()
Debug.MessageBox("В зону триггера вошли актёры")
EndFunction
Function TriggerActionStop()
Debug.MessageBox("Зону триггера покинули актёры")
EndFunction