Add a Start and End catoragory

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 = player.GetComponent<PlayerMovement> ();
    playerHealth = player.GetComponent<PlayerHealth> ();



    //Prevent the player from moving at the start of the game
    playerMovement.enabled = false;

    //Prevent the bird from moving at the start of the game
    birdMovement.enabled = false;

    //Prevent the follow camera from moving to its game position
    followCamera.enabled = false;

}

// Update is called once per frame
void Update () {
//If the game is not started and the player presses the space bar...
    if (gameStarted == false && Input.GetKeyUP (KeyCode.Space)) {  //This is the line I keep getting an error on

    //...start the game 
        StartGame();
    }
        //If the player is no longer alive....
    if (playerHealth.alive == false) {
        //...end the game.
        EndGame();

        //....increment a timer to count up to restarting....
        restartTimer = restartTimer + Time.deltaTime;

        //....and if it reaches the restart delay...
        if (restartTimer >= restartDelay) {     

            //...then reload the currrenty loaded scene
            SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);
    }

}

//Set the game state
gameStarted = true;

//Remove the start text 
    gameStateText.color = Color.clear;

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

//Allow the player to move
    birdMovement.enabled = true; 

//Allow the camera to move
    followCamera.enabled = true;

//Set the game state
    gameStarted = false;

//Show the game over Text
    gameStateText.color = Color.white;
    gameStateText.text = "Game Over!";

    //Remove the player from the game
        player.SetActive (false);