attach parent to instantiated object c#

Hello everyone I need a little help I looked around a lot of forums etc to find some answers, but everything I tried fail so...

I'm trying to attach an enemy gameobject I instantiate to a background gameobject already present in my scene. Here is the code to instantiate the gameobject

 using UnityEngine;
 using System.Collections;
 
 public class enemiescontroller : MonoBehaviour {
 
     public GameObject enemy;
     public GameObject parent;
     public Vector3 spawnvalues;
     public int hazardcount;
     public float spawnwait;
     public float startwait;
     public float wavewait;
     public GameObject player;
     
 
     // Use this for initialization
     void Start () {
 
         StartCoroutine (SpawnWaves ());
     }
 
     IEnumerator SpawnWaves () {
 
         yield return new WaitForSeconds (startwait);
         while (true) {
         for (int i = 0; i < hazardcount; i++) {
                 Vector3 spawnPosition = new Vector3 (spawnvalues.x, spawnvalues.y, spawnvalues.z);
                 //Quaternion spawnRotation = Quaternion.identity;
                 Instantiate (enemy, spawnPosition, enemy.transform.rotation);
                 enemy.transform.parent = parent.transform;
                 yield return new WaitForSeconds (spawnwait);
                 }
         yield return new WaitForSeconds (wavewait);
             //stop loop at game over
             if (player == null)
             {
                 break;}
             }
     }
 }

I don't get what I'm missing...