i cant add spawn points to my enemy prefab in the inspector or anything?

when I try to add the spawn points nothing shows up . the spawnpoints are empty gameobjects and they work on in scene enemys, but when I try to add or even drag drop to the enemy prefab in inspector I cant.

this script clearly works on in the scene gameobjects but for prefabs in the inspector it doesn't .. why is this? and do I put the spawn script on the prefab in inspector or on the gameobjects in the scene only? I was told in the past not to put the script on the prefab? 2d game

 using UnityEngine;
 using System.Collections;
 
     public class spn2 : MonoBehaviour {
         
         public GameObject EasyEnemey;
         public GameObject MediumEnemey;
         public GameObject HardEnemey;
         public Transform[] SpawnPoints;
         public float TimeBetweenSpawns;
         public int NumberOfEasyEnemiesToSpawn;
         public int NumberOfMediumEnemiesToSpawn;
         public float EasyChance;
         public float MediumChance;
         public float HardChance;
 
         private int waveNumber;
         private float spawnTimer;
         private int numberOfEasyEnemies;
         private int numberOfMediumEnemies;
         // Use this for initialization
         void Start()
         {
 
             this.spawnTimer = 3.0f;
             this.waveNumber = 0;
             float totalChance = this.EasyChance + this.MediumChance + this.HardChance;
             if(Mathf.Abs(totalChance-1.0f)>0.0001f) { 
                 Debug.LogWarning("Warning: The chances should add up to 1.0 ("+totalChance+" currently)");
             }
         }
 
         void Update()
         {
             this.spawnTimer -= Time.deltaTime;
             if(this.spawnTimer<=0.0f)
             {
                 Transform spawnPoint = this.SpawnPoints[Random.Range(0, this.SpawnPoints.Length)];
                 Vector2 spawnPos = spawnPoint.position;
                 Quaternion spawnRot = spawnPoint.rotation;
                 switch(this.waveNumber)
                 {
                 case 0:
                     Instantiate(EasyEnemey, spawnPos,spawnRot);
                     this.numberOfEasyEnemies++;
                     if(this.numberOfEasyEnemies>=this.NumberOfEasyEnemiesToSpawn)
                     {
                         this.waveNumber++;
                     }
                     break;
                 case 1:
                     Instantiate(MediumEnemey, spawnPos, spawnRot);
                     this.numberOfMediumEnemies++;
                     if (this.numberOfMediumEnemies >= this.NumberOfMediumEnemiesToSpawn)
                     {
                         this.waveNumber++;
                     }
                     break;
                 case 2:
                     float randomFloat = Random.value;
                     if(randomFloat<this.EasyChance)
                     {
                         Instantiate(EasyEnemey, spawnPos, spawnRot);
                     }
                     else if(randomFloat<this.EasyChance+this.MediumChance)
                     {
                         Instantiate(MediumEnemey, spawnPos, spawnRot);
                     }
                     else
                     {
                         Instantiate(HardEnemey, spawnPos, spawnRot);
                     }
                     break;
                 }
                 this.spawnTimer = this.TimeBetweenSpawns;
 
             //Destroy (gameObject, 2.0f);
             }
 
         }
 
     }