C# question

Hi you know i can't make character move, because c# doesn't accet word "velocity". Help me solve this problem please

public class MarioController : MonoBehaviour {

     public float maxSpeed=10f;
     bool facingRight=true;

     void Start ()
     {}

     void FixedUpdate ()
     {
         float move = Input.GetAxis ("Horizontal");

         rigidbody2D.velocity = new Vector2 (move * maxSpeed, rigidbody2D.velocity.y);

         if (move > 0 && !facingRight)
             Flip ();
         else if (move < 0 && facingRight)
             Flip ();
     }

     void Flip ()
     {
         facingRight = !facingRight;
         Vector3 theScale = transform.localScale;
         theScale.x *= -1;
         transform.localScale = theScale;

     }

 }