Gui.DrawTexture Working in Scene Mode, but not build or full game scene

Hello, I just started using Unity a couple of days ago, and I think I'm starting to get the hang of it, but not really.

Anyways, I've created this script to fade my scenes in and out, and it works when I preview the scene, but not when I build the game, or when I preview the game as a whole in the editor.

Here's my C# script:

 //this is our boolean to control when we want to fade out
     public static bool nextScene;
 
     public float duration = 0.5f;
 
     //our actual texture
     public Texture image;
 
 
     //create our GUI so that it will always draw on top
     void OnGUI () {
                 //draw above our other guis
                 GUI.depth = 0;
                 fadeIn ();
                 //now create our gui texture
                 GUI.DrawTexture (new Rect (0, 0, Screen.width, Screen.height),
                                      image);
         }
 
     // Use this for initialization
     void Start () 
     {
         nextScene = false;
     }
     
     // Update is called once per frame
     void Update () 
     {
         if (nextScene) {
             fadeOut ();
                 }
     }
 
     //function for fading in
     void fadeIn(){
         GUI.color = Color.Lerp(Color.black, Color.clear, Time.time * duration);
     }
 
     //function for fading out
 
            void fadeOut(){
         GUI.color = Color.Lerp(Color.clear, Color.black, Time.time * duration);
         }

any help is appreciated, as I've been trying to figure this out for a while. Thank you!