C# "content acceptance : Hi you know I can't make my character move in unity, 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;

		}

	}
