what's the problem with tiers ?

that's my Code:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class CarDrive : MonoBehaviour { public float WheelTorque; public float brakeTorque;

 [Space(10)]
 
 public WheelCollider FRwheel;
 public WheelCollider BRwheel;
 public WheelCollider FLwheel;
 public WheelCollider BLwheel;

 [Space (10)]

 public Transform FRwheel_transform;
 public Transform BRwheel_transform;
 public Transform FLwheel_transform;
 public Transform BLwheel_transform;


 private void FixedUpdate()
 {
     if (Input.GetKey(KeyCode.W))
     {
         DriveFoward();
     }
     if(Input.GetKeyUp(KeyCode.W))
     {
       ResetWheelRotation();
     }
     
     if (Input.GetKey(KeyCode.S))
     {
         DriveBack();
     }
     if (Input.GetKey(KeyCode.D))
     {
         TurnRight();
     }
     if(Input.GetKeyUp(KeyCode.D))
     {
       ResetWheelRotation();
     }
     if (Input.GetKey(KeyCode.A))
     {
         TurnLeft();
     }
     if(Input.GetKeyUp(KeyCode.A))
     {
       ResetWheelRotation();
     }
    if (Input.GetKey(KeyCode.Space))
     {
         Stop();
     }
     ApplyWheelRotation (FRwheel, FRwheel_transform);
     ApplyWheelRotation (BRwheel, BRwheel_transform);
     ApplyWheelRotation (FLwheel, FLwheel_transform);
     ApplyWheelRotation (BLwheel, BLwheel_transform);
 }

 private void DriveFoward()
 {
   FRwheel.motorTorque = WheelTorque;
   BRwheel.motorTorque = WheelTorque;
   FLwheel.motorTorque = WheelTorque;
   BLwheel.motorTorque = WheelTorque;
   
   FRwheel.brakeTorque = 0;
   BRwheel.brakeTorque = 0;
   FLwheel.brakeTorque = 0;
   BLwheel.brakeTorque = 0;

 }

 private void DriveBack()
 {
   FRwheel.motorTorque = -WheelTorque;
   BRwheel.motorTorque = -WheelTorque;
   FLwheel.motorTorque = -WheelTorque;
   BLwheel.motorTorque = -WheelTorque;
   
   FRwheel.brakeTorque = 0;
   BRwheel.brakeTorque = 0;
   FLwheel.brakeTorque = 0;
   BLwheel.brakeTorque = 0;

 }

 private void TurnRight()
 {
   FRwheel.steerAngle = 45;
   FLwheel.steerAngle = 45;
 }

 private void TurnLeft()
 {
   FRwheel.steerAngle = -45;
   FLwheel.steerAngle = -45;
 }
 private void ResetWheelRotation()
 {
   FRwheel.steerAngle = 0;
   FLwheel.steerAngle = 0;
 }

 private void Stop()
 {
  
  FRwheel.brakeTorque = brakeTorque;
  BRwheel.brakeTorque = brakeTorque;
  FLwheel.brakeTorque = brakeTorque;
  BLwheel.brakeTorque = brakeTorque;
          
 }

private void ApplyWheelRotation(WheelCollider WheelCollider, Transform WheelTransform) { Vector3 position; Quaternion rotation;

  WheelCollider. GetWorldPose (out position, out rotation);
  
  WheelTransform.rotation = rotation;
  WheelTransform.position = position;
 }
 

}