Game development: Unity Engine fires errors, while I do exactly as instructed in the course.

private bool gameStarted = false;
[SerializeField]
private Text gameStateText;
[SerializeField]
private GameObject player;
[SerializeField]
private BirdMovement birdMovement;
[SerializeField]
private FollowCamera followCamera;
private float restartDelay = 3f;
private float restartTimer;
private PlayerMovement playerMovement;
private PlayerHealth playerHealth;

// Use this for initialization
void Start () 
   {
    Cursor.visible = false;

    playerMovement = GetComponent<PlayerMovement> ();
    playerHealth = GetComponent<PlayerHealth> ();

    //Prevent the player from moving
    playerMovement.enabled = false ;


    //Prevent bird from moving
    birdMovement.enabled = false;

    //Prevent camera from following
    followCamera.enabled = false;
   }

// Update is called once per frame
void Update () 
  {
    // if the game is not started and the user presses space bar. the game starts. 

    if (gameStarted == false && Input.getKeyUp (KeyCode.Space)) 

     {
        StartGame();
     }

    // if player health alive is false then end the game

    if (playerHealth.alive == false)
   {
        EndGame();
  }

    // increment time
    restartTimer = restartTimer + Time.deltaTime;

    // check if timer reached the restart delay then restart the game from scrach.
    if (restartTimer >= restartDelay) 

    {
        Application.LoadLevel(Application.loadedlevel);
    }

   }

private void StartGame() 
   {

    //set the game state
    gameStarted = true;

    // remove the start text by putting it transparent .
    gameStateText.color = Color.clear;

    // allow the player to move
    playerMovement.enabled = true;

    // enable the predator . the bird to move
    birdMovement.enabled = true;

    //allow the camera to move. 
    // this gives a nice zomm out.
    followCamera.enabled = true;

   }

private void EndGame() 
    {
      // set the game state as stopped
      gameStarted =false;

      //show the text
      gameStateText.color = Color.white;

      // show GAME OVER !!
      gameStateText.text = "GAME OVER!";

      // remove the player from the game
       player.SetActive(false);
    }