Cannot cast from source type to destination type when trying to add Prefab instantiate to a list

I'm trying to fill up a list with prefabs so I have access to all my enemies in one list, but when I try to add it to a list only one enemy spawns and I get an error "Cannot cast from source type to destination type". I tried multiple solutions I found on the internet, but none of them seem to fix the problem but instead give me other errors.

The code giving me problems is the instantiate code.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class LevelManager : MonoBehaviour {
 
     public GameObject _enemyPrefab;
     private List<GameObject> _enemyList;
     //private GameObject _enemyArray[];
     private int _enemyAmount;
     private int _enemyXAmount;
     private int _enemyYAmount;
     private const int _ENEMYSPACING = 4;
     private const int _LEFTBOUND = -20;
     private const int _RIGHTBOUND = 20;
     
     //--------------------------------
     //METHODS
     //--------------------------------
     void Start () {
         //--------------------------------
         //INITIALIZE VARIABLES
         //--------------------------------
         _enemyList = new List<GameObject>();
         _enemyAmount = 0;
         _enemyXAmount = 0;
         _enemyYAmount = 0;
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
    public void SetUpLevel(int level)
     {
        //clear the still existing enemies 
        // for (int i = 0; i < _enemyList.Count; i++ )
        // {
        //     if(_enemyList[i])
        //     {
        //         Destroy(_enemyList[i], 0.0f);
        //     }
        // }
        // _enemyList.Clear();
 
 
         //different parameters depending on level
         switch (level)
         {
             case 1:
                 SetEnemyInfo(5, 5);
                 break;
             case 2:
                 SetEnemyInfo(6, 6);
                 break;
         }
 
         CreateEnemies();
 
     }
 
     void CreateEnemies()
    {
        for (int y = 0; y < _enemyYAmount; y++)
        {
            for(int x = 0; x < _enemyXAmount; x++)
            {
                Vector3 pos = new Vector3(x, 0, y) * _ENEMYSPACING;(Instantiate(_enemyPrefab, pos, Quaternion.identity));
                Transform newObject = (Transform)GameObject.Instantiate(_enemyPrefab, pos, Quaternion.identity);
                _enemyList.Add(newObject.gameObject);
 
            }
        }
    }
     void SetEnemyInfo(int x, int y)
    {
        _enemyXAmount = x;
        _enemyYAmount = y;
        _enemyAmount = x * y;
    }
 }
 

I was also wondering if I want to clear the list, do I have to check every index number if there's still a prefab in there and if there is destroy it? Or will they get destroyed when I clear my list.