Unity car simulation script

CarControl.cs

using UnityEngine;
using System.Collections;

public class CarControl : MonoBehaviour {
  // NOTE: Companion script for wheel suspensions, attach to each wheel.
  // https://gist.github.com/victorbstan/4dde0d0b4203c248423e

  // PUBLIC

  public bool driveable = false;

  // Wheel Wrapping Objects
  public Transform frontLeftWheelWrapper;
  public Transform frontRightWheelWrapper;
  public Transform rearLeftWheelWrapper;
  public Transform rearRightWheelWrapper;

  // Wheel Meshes
  // Front
  public Transform frontLeftWheelMesh;
  public Transform frontRightWheelMesh;
  // Rear
  public Transform rearLeftWheelMesh;
  public Transform rearRightWheelMesh;

  // Wheel Colliders
  // Front
  public WheelCollider wheelFL;
  public WheelCollider wheelFR;
  // Rear
  public WheelCollider wheelRL;
  public WheelCollider wheelRR;

  public float maxTorque = 20f;
  public float brakeTorque = 100f;

  // max wheel turn angle;
  public float maxWheelTurnAngle = 30f; // degrees

  // car's center of mass
  public Vector3 centerOfMass = new Vector3(0f, 0f, 0f); // unchanged

  // GUI
  //...
  public float RO_speed; // READ ONLY (Debug)
  public float RO_EngineTorque; // READ ONLY (Debug)
  public float RO_SteeringAngleFL; // READ ONLY (Debug)
  public float RO_SteeringAngleFR; // READ ONLY (Debug)
  public float RO_BrakeTorque; // READ ONLY (Debug)

  // PRIVATE

  // acceleration increment counter
  private float torquePower = 0f;

  // turn increment counter
  private float steerAngle = 0f;

  // original wheel positions
  // Front Left
  private float wheelMeshWrapperFLx;
  private float wheelMeshWrapperFLy;
  private float wheelMeshWrapperFLz;
  // Front Right
  private float wheelMeshWrapperFRx;
  private float wheelMeshWrapperFRy;
  private float wheelMeshWrapperFRz;
  // Rear Left
  private float wheelMeshWrapperRLx;
  private float wheelMeshWrapperRLy;
  private float wheelMeshWrapperRLz;
  // Rear Right
  private float wheelMeshWrapperRRx;
  private float wheelMeshWrapperRRy;
  private float wheelMeshWrapperRRz;


  void Start () {
    rigidbody.centerOfMass = centerOfMass;

    // Setup initial values

    // Front Left
    // wheelMeshWrapperFLx = frontLeftWheelWrapper.localPosition.x;
    // wheelMeshWrapperFLy = frontLeftWheelWrapper.localPosition.y;
    // wheelMeshWrapperFLz = frontLeftWheelWrapper.localPosition.z;
    // Front Right
    // wheelMeshWrapperFRx = frontRightWheelWrapper.localPosition.x;
    // wheelMeshWrapperFRy = frontRightWheelWrapper.localPosition.y;
    // wheelMeshWrapperFRz = frontRightWheelWrapper.localPosition.z;
    // Rear Left
    // wheelMeshWrapperRLx = rearLeftWheelWrapper.localPosition.x;
    // wheelMeshWrapperRLy = rearLeftWheelWrapper.localPosition.y;
    // wheelMeshWrapperRLz = rearLeftWheelWrapper.localPosition.z;
    // Rear Right
    // wheelMeshWrapperRRx = rearRightWheelWrapper.localPosition.x;
    // wheelMeshWrapperRRy = rearRightWheelWrapper.localPosition.y;
    // wheelMeshWrapperRRz = rearRightWheelWrapper.localPosition.z;
  }


  // Visual updates
  void Update () {
    if (! driveable) {
      return;
    }

    // SETUP WHEEL MESHES

    // Turn the mesh wheels
    frontLeftWheelWrapper.localEulerAngles = new Vector3(0, steerAngle, 0);
    frontRightWheelWrapper.localEulerAngles = new Vector3(0, steerAngle, 0);

    // Wheel rotation
    frontLeftWheelMesh.Rotate(0, wheelFL.rpm / 60 * 360 * Time.deltaTime, 0);
    frontRightWheelMesh.Rotate(0, wheelFR.rpm / 60 * 360 * Time.deltaTime, 0);
    rearLeftWheelMesh.Rotate(0, wheelRL.rpm / 60 * 360 * Time.deltaTime, 0);
    rearRightWheelMesh.Rotate(0, wheelRR.rpm / 60 * 360 * Time.deltaTime, 0);

    // Audio
    audio.pitch = (torquePower / maxTorque) + 0.5f;
  }

  // Physics updates
  void FixedUpdate () {
    if (! driveable) {
      return;
    }

    // CONTROLS - FORWARD & RearWARD
    if ( Input.GetKey(KeyCode.Space) ) {
      // BRAKE
      torquePower = 0f;
      wheelRL.brakeTorque = brakeTorque;
      wheelRR.brakeTorque = brakeTorque;
    } else {
      // SPEED
      torquePower = maxTorque * Mathf.Clamp( Input.GetAxis("Vertical"), -1, 1 );
      wheelRL.brakeTorque = 0f;
      wheelRR.brakeTorque = 0f;

    }
    // Apply torque
    wheelRR.motorTorque = torquePower;
    wheelRL.motorTorque = torquePower;

    // Debug.Log(Input.GetAxis("Vertical"));
    Debug.Log("torquePower: " + torquePower);
    Debug.Log("brakeTorque RL: " + wheelRL.brakeTorque);
    Debug.Log("brakeTorque RR: " + wheelRR.brakeTorque);
    Debug.Log("steerAngle: " + steerAngle);

    // CONTROLS - LEFT & RIGHT
    // apply steering to front wheels
    steerAngle = maxWheelTurnAngle * Input.GetAxis("Horizontal");
    wheelFL.steerAngle = steerAngle;
    wheelFR.steerAngle = steerAngle;

    // Debug info
    RO_BrakeTorque = wheelRL.brakeTorque;
    RO_SteeringAngleFL = wheelFL.steerAngle;
    RO_SteeringAngleFR = wheelFR.steerAngle;
    RO_EngineTorque = torquePower;

    // SPEED
    // debug info
    RO_speed = rigidbody.velocity.magnitude;

    // KEYBOARD INPUT

    // FORWARD
    if ( Input.GetKey(KeyCode.W) ) {
      // Debug.Log("FORWARD");
    }

    // BACKWARD
    if ( Input.GetKey(KeyCode.S) ) {
      // Debug.Log("BACKWARD");
    }

    // LEFT
    if ( Input.GetKey(KeyCode.A) ) {
      // Debug.Log("LEFT");
    }

    // RIGHT
    if ( Input.GetKey(KeyCode.D) ) {
      // Debug.Log("RIGHT");
    }

    // BRAKE
    if ( Input.GetKey(KeyCode.Space) ) {
      // Debug.Log("SPACE");
    }
  }
}

I want to change the controls to a screen button, how can I convert this to a button input, for example I press gas button and it will move forward, what function makes it move forward when w is pressed and what function makes it go left and right when a and d is pressed, the getKeycode function has nothing inside...

Hello everyone! I don't know why I am getting the following issue whenever I am entering play mode after assigning the scrip to my car: -
" ArgumentNullException: Value cannot be null.
Parameter name: source UnityEngine.AudioSource.set_pitch (System.Single value) (at <7117d168a9ec4e518e0de7d9c98bd09a>:0)
CarMovement.Update () (at Assets/Scripts/CarMovement.cs:125) "

I cannot find any fix for it (I am a beginner). I have assigned the desired objects to every variable slot. Please Help.
Just to be informed I am using this ( https://assetstore.unity.com/packages/3d/vehicles/land/free-sci-fi-car-184607 ) car for my project and have assigned the wheel colliders to its wheels.

Please Help.

My car gets sent flying into the air, how do i fix this?

Add rigidbody to it.

What do you mean by "Wheel Wrapper"?