issue with SceneManager.LoadScene

hello there , i have a main scene at the index 0 , i created more scenes that are just enemies that spawn when all the previous enemies are dead . i try to add them with SceneManager.LoadScene(scene, LoadSceneMode.Additive) and i don't know why it doesn't work . here is more code for you to understand

 using UnityEngine.SceneManagement;
 
 public int currentScene;
 
  void Start () {
 
         currentScene = 1;     
 }
  public void nextLevel ()
     {
         
         SceneManager.LoadScene(currentScene , LoadSceneMode.Additive);       
         Time.timeScale = 0;
         currentScene ++;
         lvlNumber++;
         displayLvl();
         StartCoroutine(startPlay());
         sound.startLevelSound(.8f);
     }