RigidBody Physics Flight Simulator Script Help

Hi, I am new to Unity, and I am currently working on a Rigidbody-based flight physics script. For the rotation, pitching, yawing of the plane, I am trying to use rigidbody.AddForceAtPosition. I save this script, but unity replies with this error saying: "Assets/Standard Assets/Scripts/General Scripts/FLightSim.js(21,49): BCE0023: No appropriate version of 'UnityEngine.Rigidbody.AddForceAtPosition' for the argument list '(UnityEngine.Vector3, UnityEngine.Transform, System.Object)' was found.”

I have no idea what is wrong with my script. Can somebody please give me a basic idea of principles of Rigid Body scripting please?

This is my current script:

 var MaxThrottleStrength : float;
 var LiftRatio : float;
 var wingElevatorRight : Transform;
 var wingElevatorLeft : Transform;
 var ElevatorRight : Transform;
 var ElevatorLeft : Transform;
 var EngineStrength : float;
 var ElevatorStrength : float;
 var rotatestrength : float;
 
 var EngineIncreaseSpeed : float;
 private var Lift : float;
 
 function Start(){
     EngineStrength = 0;
 }
 
 function FixedUpdate(){
     Lift = EngineStrength/10;
     gameObject.rigidbody.AddRelativeForce ( Vector3.forward * EngineStrength);
     gameObject.rigidbody.AddForceAtPosition ( Vector3.up * rotatestrength, wingElevatorRight, ForceMode.relativeForce );
     gameObject.rigidbody.AddForceAtPosition ( Vector3.up * rotatestrength * -1, wingElevatorLeft, ForceMode.relativeForce );
     gameObject.rigidbody.AddRelativeForce ( Vector3.up * Lift );
     gameObject.rigidbody.AddForceAtPosition ( Vector3.up * ElevatorStrength, ElevatorRight, ForceMode.relativeForce );
     gameObject.rigidbody.AddForceAtPosition ( Vector3.up * ElevatorStrength, ElevatorLeft, ForceMode.relativeForce );
 }
 
 function Update(){
     if (Input.GetButton ("Vertical1")){
         EngineStrength = EngineStrength + EngineIncreaseSpeed;
     }
     if (Input.GetButton ("Vertical2")){
         ElevatorStrength = -20;
     }
     if (Input.GetButton ("Horizontal")){
         rotatestrength = 20;
     }
     
 }