how to add prefab to gameobject?

I'm trying to have 'foundation' as a child of 'empty' but with my code it doesn't do that but it also gives no errors, what am I doing wrong?

 public List<GameObject> foundations = new List<GameObject>();
 public List<GameObject> levels = new List<GameObject>();
 public List<GameObject> roofs = new List<GameObject>();

 public BuildSystem buildSystem;

 GameObject empty;
 
 private int x = 0;


 public void CreateNewBuilding()
 {
     x++;
     empty = new GameObject("Building" + x);
 }

 public void StopBuilding()
 {
     Destroy(empty);
     x--;
 }

 public void AddFoundation(GameObject foundation)
 {
     foundations.Add(foundation);
     buildSystem.NewBuild(foundation);
     foundation.transform.SetParent(empty.transform);
 }