There is no 'Rigidbody' attached to the "Cube" game object, but a script is trying to access it,There is no Rigidbody attached to the Cube game object.

MissingComponentException: There is no ‘Rigidbody’ attached to the “Cube” game object, but a script is trying to access it.
You probably need to add a Rigidbody to the game object “Cube”. Or your script needs to check if the component is attached before using it.

I receive this error message whenever i try to run this code:

#pragma strict

var rotationSpeed= 100;

function Update () 
{

var rotation : float = Input.GetAxis ("Horizontal") * rotationSpeed;
rotation *= Time.deltaTime;
GetComponent.().AddRelativeTorque (Vector3.back * rotation);

}

How can i fix it???
HOW?

Add a RigidBody to your GameObject. Pretty simple. If you have any trouble, please reply this answer.