My Drag Selection is not working!! Need Help for an RTS Game!

Hi, is there anyone that knows why my selection in bounds not working??? The Rect GUI when dragging have displayed but it won't select my units

I am using this tutorial: How To Build an RTS Game

and

Here's the code

     public static Texture2D WhiteTexture
     {
         get
         {
             if (_whiteTexture == null)
             {
                 _whiteTexture = new Texture2D(1, 1);
                 _whiteTexture.SetPixel(0, 0, Color.white);
                 _whiteTexture.Apply(); 
             }
 
             return _whiteTexture;
         }
     }
 
     public static void DrawScreenRect(Rect rect, Color color)
     {
         GUI.color = color;
         GUI.DrawTexture(rect, WhiteTexture);
     }
 
     public static void DrawScreenRectBorder(Rect rect, float thickness, Color color)
     {
         //Top
         DrawScreenRect(new Rect(rect.xMin, rect.yMin, rect.width, thickness), color);
         //Bottom
         DrawScreenRect(new Rect(rect.xMin, rect.yMax - thickness, rect.width, thickness), color);
         //Left
         DrawScreenRect(new Rect(rect.xMin, rect.yMin, thickness, rect.height), color);
         //Right
         DrawScreenRect(new Rect(rect.xMax - thickness, rect.yMin, thickness, rect.height), color);
     }
 
     public static Rect GetScreenRect(Vector3 screenPos1, Vector3 screenPos2)
     {
         screenPos1.y = Screen.height - screenPos1.y;
         screenPos2.y = Screen.height - screenPos2.y;
 
         Vector3 bR = Vector3.Max(screenPos1, screenPos2);
         Vector3 tL = Vector3.Min(screenPos1, screenPos2);
 
         return Rect.MinMaxRect(tL.x, tL.y, bR.x, bR.y);
     }
 
     public static Bounds GetVPBounds(Camera cam, Vector3 screenPos1, Vector3 screenPos2)
     {
         Vector3 pos1 = cam.ScreenToViewportPoint(screenPos1);
         Vector3 pos2 = cam.ScreenToViewportPoint(screenPos2);
 
         Vector3 min = Vector3.Min(pos1, pos2);
         Vector3 max = Vector3.Max(pos1, pos2);
 
         min.z = cam.nearClipPlane;
         max.z = cam.farClipPlane;
 
         Bounds bounds = new Bounds();
         bounds.SetMinMax(min, max);
 
         return bounds;  
     }
 
     Update()
     {
             if(Input.GetMouseButtonDown(0))
             {
                 mousePos = Input.mousePosition;
 
                 Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);        
 
                 if(Physics.Raycast(ray, out hit))
                 {
                     LayerMask layerHit = hit.transform.gameObject.layer;
 
                     switch (layerHit.value)
                     {
                         case 9:
                             if(!selectedUnits.Contains(hit.transform))
                             SelectUnit(hit.transform);
                             break;
                         default:
                             DeselectUnit();
                             isDragging = true;
                             break;
                     }
                 }        
             }
 
             if(Input.GetMouseButtonUp(0))
             {
                 foreach(Transform child in map)
                 {
                     if(child.name == "3D Markers")
                     foreach(Transform units in child)
                     {
                         if(IsWithinSelectionBounds(units))
                         SelectUnit(units);
                     }
                 }
                 isDragging = false;
             }  
     }  
     
     private void OnGUI()
     {
         if (isDragging)
         {
             Rect rect = GetScreenRect(mousePos, Input.mousePosition);
             DrawScreenRect(rect, new Color(0f, 0f, 0f, 0.25f));
             DrawScreenRectBorder(rect, 3, Color.blue);
         }
     }
 
     private bool IsWithinSelectionBounds(Transform tf)
     {
         if(isDragging)
         {
             return false;
         }
 
         Camera cam = Camera.main;
         Bounds vpBounds = GetVPBounds(cam, mousePos, Input.mousePosition);
         return vpBounds.Contains(cam.WorldToViewportPoint(tf.position));
     }