FPS Crouch script error, I have this script but it keeps giving me this error, Assets\Scripts\Crouch

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Crouch : MonoBehaviour
{
      public CharacterController characterController;

        

     

    void Start()
    {
        characterController = GameObject.GetComponent<CharacterController>();
    }

  
    void Update()
    {
        if (Input.GetKey(KeyCode.LeftControl))
        {
          characterController.height = 1f;
        }
          else
          {
              characterController.height = 3.8f;
          }


       
    }
}

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http://plbm.com/?p=220