Adding Main Menu to Unity's official Roguelike tutorial

I've been trying to add a main menu to the game from unity's Roguelike tutorial but every time I complete a level the menu opens again. What should I do to fix this?

GameManager script

 using System;
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 
 public class GameManager : MonoBehaviour
 {
     public static GameManager instance = null;
 
     public BoardManager boardScript;
     public int playerFoodPoints = 100;
     [HideInInspector] public bool playersTurn = true;
     public float turnDelay = .1f;
     public float levelStartDelay = 2f;
 
     private int level = 0;
     private List<Enemy> enemies;
     private bool enemiesMoving;
     private Text levelText;
     private GameObject levelImage;
     private bool doingSetup;
 
     void Awake()
     {
         if (instance == null) {
             instance = this;
         } else if (instance != this) {
             Destroy(gameObject);
         }
 
         DontDestroyOnLoad(gameObject);
 
         enemies = new List<Enemy>();
 
         boardScript = GetComponent<BoardManager>();
         
     }
 
     void InitGame()
     {
         doingSetup = true;
 
         levelImage = GameObject.Find("LevelImage");
         levelText = GameObject.Find("LevelText").GetComponent<Text>();
         levelText.text = "Dia " + NumberToWords(level);
         levelImage.SetActive(true);
 
         Invoke("HideLevelImage", levelStartDelay);
 
         enemies.Clear();
 
         boardScript.SetupScene(level);
     }
 
     private void HideLevelImage()
     {
         levelImage.SetActive(false);
         doingSetup = false;
     }
 
     void OnLevelFinishedLoading(Scene scene, LoadSceneMode mode)
     {
             level++;
             InitGame();
     }
 
     void OnEnable()
     {
         SceneManager.sceneLoaded += OnLevelFinishedLoading;
     }
 
     void OnDisable()
     {
         SceneManager.sceneLoaded -= OnLevelFinishedLoading;
     }
 
     void Update()
     {
         if (playersTurn || enemiesMoving || doingSetup) {
             return;
         }
 
         StartCoroutine(MoveEnemies());
     }
 
     public void AddEnemyToList(Enemy script)
     {
         enemies.Add(script);
     }
 
     public void GameOver()
     {
         levelText.text = "Sobreviveu " + NumberToWords(level).ToLower() + " " + ((level <= 1) ? "dia" : "dias");
         levelImage.SetActive(true);
 
         enabled = false;
     }
 
     IEnumerator MoveEnemies()
     {
         enemiesMoving = true;
 
         yield return new WaitForSeconds(turnDelay);
 
         if (enemies.Count == 0) {
             yield return new WaitForSeconds(turnDelay);
         }
 
         for (int i = 0; i < enemies.Count; i++)
         {
             enemies[i].MoveEnemy();
 
             yield return new WaitForSeconds(enemies[i].moveTime);
         }
             
         enemiesMoving = false;
 
         playersTurn = true;
     }
 
     private static string NumberToWords(int number)
     {
         if (number == 0)
             return "Zero";
 
         if (number < 0)
             return "Menos " + NumberToWords(Math.Abs(number));
 
         string words = "";
 
         if ((number / 1000000) > 0)
         {
             words += NumberToWords(number / 1000000) + " Milhão ";
             number %= 1000000;
         }
 
         if ((number / 1000) > 0)
         {
             words += NumberToWords(number / 1000) + " Milhar ";
             number %= 1000;
         }
 
         if ((number / 100) > 0)
         {
             words += NumberToWords(number / 100) + " Centena ";
             number %= 100;
         }
 
         if (number > 0)
         {
             if (words != "")
                 words += "e ";
 
             var unitsMap = new[] { "Zero", "1", "2", "3", "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15", "16", "17", "18", "19" };
             var tensMap = new[] { "Zero", "10", "20", "30", "40", "50", "60", "70", "80", "90" };
 
             if (number < 20)
                 words += unitsMap[number];
             else
             {
                 words += tensMap[number / 10];
                 if ((number % 10) > 0)
                     words += "-" + unitsMap[number % 10];
             }
         }
 
         return words;
     }
 }
 

Menu script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 
 public class MainMenu : MonoBehaviour {
 
     public void PlayGame ()
     {
         SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex + 1);
     }
 
     public void QuitGame ()
     {
         Debug.Log("SAIR!");
         Application.Quit();
     }
 }