Strange errors with GUI.skin

Hi i was making a GUI script, and stumbled upon an error. Due to little programming experience i have no idea how to solve this:

MissingFieldException: Field 'GUI.skin' not found. Boo.Lang.Runtime.DynamicDispatching.PropertyDispatcherFactory.FindExtension (IEnumerable`1 candidates) Boo.Lang.Runtime.DynamicDispatching.PropertyDispatcherFactory.Create (SetOrGet gos) Boo.Lang.Runtime.DynamicDispatching.PropertyDispatcherFactory.CreateSetter () Boo.Lang.Runtime.RuntimeServices.DoCreatePropSetDispatcher (System.Object target, System.Type type, System.String name, System.Object value) Boo.Lang.Runtime.RuntimeServices.CreatePropSetDispatcher (System.Object target, System.String name, System.Object value) Boo.Lang.Runtime.RuntimeServices+c_AnonStorey16.<>m_C () Boo.Lang.Runtime.DynamicDispatching.DispatcherCache.Get (Boo.Lang.Runtime.DynamicDispatching.DispatcherKey key, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.String cacheKeyName, System.Type[] cacheKeyTypes, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.Object[] args, System.String cacheKeyName, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.SetProperty (System.Object target, System.String name, System.Object value) GUI.OnGUI () (at Assets/Scripts/Player/GUI.js:8)

This is the script:

 var GUIskin:GUISkin;
 var healthMeter:Texture2D[];
 var secondaryResourceMeter:Texture2D[];
 
 function OnGUI(){
     GUI.skin = GUIskin;
     if(healthMeter.Length == 10){
         GUI.Label(Rect(10, 10, Screen.width / 2 - 5, 50), healthMeter[10 - 1], ScaleMode.ScaleToFit, true, 0);
     }else{
         Debug.LogError("More health Textures are needed, GUI");
     }
     
 }