Time does not start counting down when need to

Hello, im making 2d mobile game and i came to problem. So my game starts on click and timers and every element shows on that click and needs to start when i touch the screen, and here is the problem, my timer of game doesnt start countdown on click it just shows himself, i know this code isnt right i tried to fix it with another method and all kinds of stuff, but no luck. Btw my timer is in update function. Here is my code:

using System.Collections; using System.Collections.Generic; using UnityEngine; using UnityEngine.UI; using System; using UnityEngine.SceneManagement;

public class TimerIIgra : MonoBehaviour {

 //za tajmer 30s
 private float timerIgre = 30f;
 [SerializeField] private Text textTimeraIgre;
 [SerializeField] private Text textFinishedScore;
 [SerializeField] private Text tapCounterText;
 public int tapCounter = 0;

 [SerializeField] public GameObject retryButton;
 [SerializeField] public GameObject mainMenuButton;

 //public countDownTimer countDownTimerScript;

 public bool gameStart = false;
 //public bool gameStarted = true;

 public Animator animator;

 void Start () {
     //textTimeraIgre = GetComponent<Text>();
     textTimeraIgre.text = "";
     tapCounterText.text = "";
     textFinishedScore.text = "";
     retryButton.SetActive(false);
     mainMenuButton.SetActive(false);
     animator = GetComponent<Animator>();
     //if (Input.GetMouseButtonDown(0))
     //{
         //gameStart = true;
     //}
     //StartGame();
 }
 public void Retry()
 {
     SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);
 }

 public void BackToMenu()
 {
     SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex - 1);
 }

 /*public void StartGame()
 {
     if (Input.GetMouseButtonDown(0))
     {
         gameStart = false;
     }
 }*/

 void Update()
 {
     //if (Input.GetMouseButtonDown(0))
     //{
         //gameStart = true;
         if (gameStart == true)
         {
             textTimeraIgre.text = "30";
             timerIgre -= Time.deltaTime;
             textTimeraIgre.text = timerIgre.ToString("f0");

             if (Input.GetMouseButtonDown(0))
             {
                 animator.SetBool("isPressed", true);
                 tapCounter++;
                 tapCounterText.text = tapCounter.ToString();
             }
             else
             {
                 animator.SetBool("isPressed", false);
             }

             if (timerIgre <= 0)
             {
                 tapCounterText.text = "";
                 gameStart = false;
                 textTimeraIgre.text = timerIgre.ToString("Time's Up!");
                 textFinishedScore.text = "Score: " + tapCounter.ToString();

                 //FindObjectOfType<GameManager>().EndGame();

                 retryButton.SetActive(true);
                 mainMenuButton.SetActive(true);
             }
         }
     //}
 }

}