Ball doesn't roll in roll-a-ball for mac

I am new to Unity and followed the tutorials to create the "roll-a-ball" game; However the ball and other events do not function. An error message shows up to correct the errors in the script but the compiler doesn't find any error:

/Users/elliott/Desktop/Capture d’écran 2016-11-20 à 11.22.10.png

and this is the script for the ball :

using UnityEngine; using System.Collections;

public class PlayerController : MonoBehaviour {

 public float speed;

 private Rigidbody rb;

 void Start ()
 {
     rb = GetComponent<Rigidbody>();
 }

 void FixedUpdate ()
 {
     float moveHorizontal = Input.GetAxis ("Horizontal");
     float moveVertical = Input.GetAxis ("Vertical");

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

     rb.AddForce (movement * speed);
 }

}

thank you in advance for your answers !