How do I load scene using scene manager Set Active Scene build index / ManagerScenes are not loading Build index

I only gotten one scene to load . I need one more scene . The scene manager builder index is not working : I have a picture of what I scene I want to load :

Here is my code :

 using UnityEngine; 
 using System.Collections; 
 using UnityEngine.UI; 
 using UnityEngine.SceneManagement;
 
 public class TestScript : MonoBehaviour 
 { 
     public MyClockScript myClock; 
     public ScoreManagerScript scoremanager; 
     public int scoreToReach = 99; // change this value to what you want 
     public string nextScene = "FY"; // change this value to what you want
 
     
     void Update () 
      {
            if (myClock.m_leftTime <= 0)
                {
                if ((ScoreManagerScript.score >= scoreToReach) && (nextScene != ""))
                       {
                       SceneManager.LoadScene(nextScene);
                       }
                else
                      {
                       
                       int gy = SceneManager.GetActiveScene().buildIndex;
 
  
                      }
                 }
      }
  }