Event System, OnClick function

Hi, I'va a problem to change scene upon clicking the button. I use countdown so when user click all the button before the timer end, it will pop out congrats message. And if the timer end before doing so, 'game over' message will pop out. By the way, I'm new to Unity. Hope you guys can help me to sort it out. Thanks in advance.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 
 
 public class StartDown : MonoBehaviour
 {
     int countDownStartValue = 20;
     public Text timerUI;
     public GameObject gameOver;
     public GameObject Menu;
     public GameObject ViewCreature;
     public GameObject Exit;
     public GameObject wonM;
 
     public bool won = false;
     public Button Trash1;
     public Button Trash2;
     public Button Trash3;
     public Button Trash4;
 
 
     void Start()
     {
         countDownTimer();
         gameOver.SetActive(false);
         Menu.SetActive(false);
         ViewCreature.SetActive(false);
         Exit.SetActive(false);
         wonM.SetActive(false);
 
         Trash1.onClick.AddListener(TaskOnClick);
         Trash2.onClick.AddListener(TaskOnClick);
         Trash3.onClick.AddListener(TaskOnClick);
         Trash4.onClick.AddListener(TaskOnClick);
     }
 
     public void TaskOnClick()
     {
         if (won)
         {
             won = true;
             Menu.SetActive(true);
             ViewCreature.SetActive(true);
             Exit.SetActive(true);
             wonM.SetActive(true);
         }
         else
         {
             gameOver.SetActive(true);
 
         }
     }
 
 
     void countDownTimer()
     {
         if (countDownStartValue > -1)
         {
             timerUI.text = " " + countDownStartValue;
             countDownStartValue--;
             Invoke("countDownTimer", 1.0f);
         }
 
         else
         {
             gameOver.SetActive(true);
             FindObjectOfType<Sound>().DeathSound();
             Menu.SetActive(true);
             ViewCreature.SetActive(true);
             Exit.SetActive(true);
             wonM.SetActive(false);
         }
     
     }
         
 
 }