I can't move camera with my character.

This is my first time using Unity and I stuck at very basic. I can't move camera with my character. I follow one Youtuber's videos, Quill18. I enter same code with him. But result is not the same. Here is the code:

 using UnityEngine;
 using System.Collections;
 
 public class CameraTracksPlayer : MonoBehaviour {
     
     Transform player;
     float offsetX;
     Vector3 pos;
     
     // Use this for initialization
     void Start () {
         GameObject player_go = GameObject.FindGameObjectWithTag("Player");
         if(player_go == null) {
             Debug.LogError("Couldn't find an object with tag 'Player'!");
             return;
         }
         player = player_go.transform;
         offsetX = (transform.position.x - player.position.x);
     }
     
     // Update is called once per frame
     void Update () {
         if(player != null) {
             pos = transform.position;
             pos.x = (player.position.x + offsetX);
             transform.position = pos;
         }
     }
 }

By the way, I have tried several different methods from other people's Q&As but nothing worked.