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 :smiley:

@Aqshalkun
timeleft += 20;
use this instead