I would love to know how to keep spawning enemies because when i shoot them they don't spawn. The cause is that the prefab is missing as it says heres my code

 var zombiePrefab : GameObject;
 var zombiesSpawned : int;
 var player : GameObject;
 var spawn : boolean;
 var minWait : int;
 var maxWait : int;
 var waitTime : int;
 
 function Start()
 {
 minWait = 1;
 maxWait = 1;
 waitTime = Random.Range(minWait, maxWait);;
 spawn = true;
 }
 
 
 function Update()
 {
 if(spawn)
 {
 
 Spawn();
 
 }
 
 }
 
 
 function Spawn()
 {
 
 //spawn
 Instantiate(zombiePrefab, transform.position, transform.rotation);//spawn at spawner location
 zombiesSpawned += 1;
 NewWaitTime();
 spawn = false;
 
 SetSpawn();
 }
 
 function SetSpawn()
 {
     yield WaitForSeconds(waitTime);
     spawn = true;
 }
 
 
 function NewWaitTime()
 {
 waitTime = Random.Range(minWait, maxWait);
 }