problem with linking reference with Gameobject. ,my reference is not link with GameObject script

my level complete(panel) reference is not linking with GameObject & it says default references will only applied in edit mode. take a look in my game object coding.

 using System;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 public class GameObject : MonoBehaviour
 {
     bool GameHasEnded = false;
 
     public float restartDelay = 2f;
     public GameObject completelevelUI;
     public void CompleteLevel()
     {
         completelevelUI.SetActive(true);
     }
 
     private void SetActive(bool v)
     {
         throw new NotImplementedException();
     }
 
     public void EndGame()
     {
         if (GameHasEnded == false)
         {
             GameHasEnded = true;
             Debug.Log("game over");
             Invoke("Restart",restartDelay);
         }
        
     }    void Restart()
     {
         SceneManager.LoadScene(SceneManager.GetActiveScene().name);
     }
 }

,when i link panel(level complete) to the GameObject script then it shows None(gameobject),
when i dragged level complete (panel) to the game object script of completelevelUI it shows None(gameobject) I attached my code file ,please check it & send me correction

using System; using UnityEngine; using UnityEngine.SceneManagement; public class GameObject : MonoBehaviour { bool GameHasEnded = false; public float restartDelay = 2f; public GameObject completelevelUI; public void CompleteLevel() { completelevelUI.SetActive(true); } private void SetActive(bool v) { throw new NotImplementedException(); } public void EndGame() { if (GameHasEnded == false) { GameHasEnded = true; Debug.Log("game over"); Invoke("Restart",restartDelay); } } void Restart() { SceneManager.LoadScene(SceneManager.GetActiveScene().name); } }