getting gui error you can only call functions from inside OnGui.

I am getting the following error when i execute this script . plz help!:-

You can only call GUI functions from inside OnGUI. UnityEngine.GUIUtility.CheckOnGUI () (at C:/BuildAgent/work/aeedb04a1292f85a/artifacts/EditorGenerated/GUIUtility.cs:229) UnityEngine.GUI.get_skin () (at C:/BuildAgent/work/aeedb04a1292f85a/artifacts/EditorGenerated/GUI.cs:54) UnityEngine.GUILayout.Label (System.String text, UnityEngine.GUILayoutOption[] options) (at C:/BuildAgent/work/aeedb04a1292f85a/artifacts/EditorGenerated/GUILayout.cs:17) flap.OnGUI () (at Assets/scripts/flap.cs:61) flap.OnCollisionEnter2D (UnityEngine.Collision2D other) (at Assets/scripts/flap.cs:28)

script:-

  using UnityEngine;
     using System.Collections;
     public class flap: MonoBehaviour
   
     public void OnGUI () 
     {
             Debug.Log ("ok");
         if(dead)
         {
             Debug.Log("working");
             Time.timeScale=0;
             GUI.color = Color.white;
             GUILayout.Label (" Score: " + score.ToString ());
         }
         }
     }