Unable to add object to Gameobject Field

I'm attempting to transform.SetParent an instantiated prefab to my player during the game, and I'm pretty sure I have the code worked out for it, only, when I attempt to add the Player Object (located in the Hierarchy) to the Game Object field on the prefab, it will not let me. I'm unsure why this is taking place (bit of a newbie, so excuse my inexperience). Attached is the relevant bit of code, located in the object's script.

  public GameObject Cube;
     public void OnCollisionStay(Collision collision)
     {
         if (collision.gameObject.CompareTag("Player"))
         { Cube.transform.SetParent(this.transform, false); }
     }

Thanks!