How to make bonus time in Unity C#

I want to ask about how to make a bonus time in my game when player hit the time bonus item. Here is my code
CountDownTimer

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class countDownTimer : MonoBehaviour {
     public bool lol;
     public int timeLeft = 5;
     public Text countdownText;
     public GameObject menang, kalah, reset, kanan, kiri, atas, bola, pause;
     // Use this for initialization
 
 
     void Start()
     {
         StartCoroutine("LoseTime");
         lol = false;
     }
 
     // Update is called once per frame 
     void Update()
     {
         countdownText.text = ("Time Left = " + timeLeft);
         if (menang.active == true)
         {
             StopCoroutine("LoseTime");
             countdownText.text = ("Score = " + timeLeft);
         }
         if (timeLeft <= 0)
         {
             StopCoroutine("LoseTime");
             countdownText.text = ("Time Left = " + timeLeft);
             kalah.SetActive(true);
             reset.SetActive(true);
             atas.SetActive(false);
             kanan.SetActive(false);
             kiri.SetActive(false);
             bola.SetActive(false);
             pause.SetActive(false);
         }
     }
 
     void OnTriggerEnter2D(Collider2D other)
     {
         if (other.transform.tag == "Player")
         {
             menang.SetActive(true);
             //StopCoroutine("LoseTime");
         }
     }
 
 
     IEnumerator LoseTime()
     {
 
         while (true)
         {
             yield return new WaitForSeconds(1);
             timeLeft--;
         }
     }
 
     
 }
 

Item

  void OnTriggerEnter2D(Collider2D other)
     {
         if (other.transform.tag == "Player")
         {
             timeleft + 20;
         }
     }

and the problem is, when I add 20 time in my timeleft, the console says that Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement.

Thanks before :D