Object Reference Required to access non-static member 'UnityEngine.RigidBody.AddRelativeTorque'

I am attempting to create a rotation with rigidbody’s add relative torque function. I am getting the error: An Object Reference Required to access non-static member ‘UnityEngine.RigidBody.AddRelativeTorque’ and I don’t know how to fix this. This is my code:

public class NewRotation : MonoBehaviour {
	 void FixedUpdate() {
		if (Input.GetButtonDown("Horizontal(-)")) {
			Rigidbody.AddRelativeTorque(0, 0, -15);
		}
		if (Input.GetButtonDown("Horizontal(+)")) {
			Rigidbody.AddRelativeTorque(0, 0, 15);
	}
	}
}

You want ‘rigidbody’ with a lower case ‘r’. ‘Rigidbody’ is the class. Example:

 rigidbody.AddRelativeTorque(0, 0, -15);