다시하기를 누르면 안된다는데

해결 방법을 모르겠습니다 ㅠㅠ

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.SceneManagement;


public class gameManager : MonoBehaviour


{
    public GameObject square;
    public GameObject endPanel;
    public Text timeTxt;
    public Text thisScoreTxt;
    float alive = 0f;
    bool isRunning = true;
    
    public static gameManager I;


    void Awake()
    {
        I = this;
    }
    // Start is called before the first frame update
    void Start()
    {
        Time.timeScale = 1.0f;
        InvokeRepeating("makeSquare", 0.0f, 0.5f);
    }


    // Update is called once per frame
    void Update()
    {
        if (isRunning)
        {
            alive += Time.deltaTime;
            timeTxt.text = alive.ToString("N2");
        }
    }


    void makeSquare()
    {
        Instantiate(square);
    }


    public void GameOver()
    {
        isRunning = false;
        Time.timeScale = 0f;
        endPanel.SetActive(true);
        thisScoreTxt.text = alive.ToString("N2");
    }


    public void retry()
{
    SceneManager.LoadScene("MainScene");
}
}