main camera doesn't has rigid body

// gather components to the player gameobject
void Start () {
    playerAnimator = GetComponent<Animator> ();
    playerRigidbody = GetComponent<Rigidbody> ();

}

// Update is called once per frame
void Update () {
    // gather imput from the keyboard
    moveHorizontal = Input.GetAxisRaw ("Horizontal");
    moveVertical = Input.GetAxisRaw ("Vertical");

    movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
    }


void FixedUpdate () { 

    // if the players movement vector is not equal to zero...
    if (movement != Vector3.zero) {


        // ...then create a target rotation based on the movement vector... 
        Quaternion targetRotation = Quaternion.LookRotation(movement, Vector3.up);

        //... and create another rotation that moves from the current rotation to the target rotation...
        Quaternion newRotation = Quaternion.Lerp (playerRigidbody.rotation, targetRotation, turningSpeed * Time.deltaTime );

        //... and change the players rotation to the new incremental rotation...
        playerRigidbody.MoveRotation(newRotation);

        //... then play the jump animation.
        playerAnimator.SetFloat ("Speed", 3f);
    } else {
    // otherwise, dont play the jump animation
        playerAnimator.SetFloat ("Speed", 0f);

    }

}