no extension method `GetTracker' - Vuforia + Unity

I have been following various tutorials on vuforia on building a recognition app, however I had a problem with one part of the code that I can't seem to fix.

My code gives me an error like this:

Type UnityEngine.GameObject' does not contain a definition for GetTracker' and no extension method GetTracker' of type UnityEngine.GameObject' could be found.

Is there any reason the GetTracker doesn't work? I'm new to this coding thing and I don't know what could be the problem.

Here is the whole code, in case maybe I missed something.

 public class SimpleCloudHandler : MonoBehaviour, ICloudRecoEventHandler {

 private CloudRecoBehaviour mCloudRecoBehaviour;

 private bool mIsScanning = false;

 private string mTargetMetadata = "";

 private ImageTracker mImageTracker;

 public TrackerManager mTrackerManager;

 public ImageTargetBehaviour ImageTargetTemplate;

 // Use this for initialization

 void Start ()

 {

 // register this event handler at the cloud reco behaviour

 mCloudRecoBehaviour = GetComponent<CloudRecoBehaviour>();
 
 if (mCloudRecoBehaviour)

 {

  mCloudRecoBehaviour.RegisterEventHandler(this);

 }

 }

 public void OnInitialized() {

 Debug.Log ("Cloud Reco initialized");

 }

 public void OnInitError(TargetFinder.InitState initError) {

 Debug.Log ("Cloud Reco init error " + initError.ToString());

 }

 public void OnUpdateError(TargetFinder.UpdateState updateError) {

 Debug.Log ("Cloud Reco update error " + updateError.ToString());

 }

 public class ImageTracker

 {

 }

 public class TrackerManager

 {

 public GameObject Instance;

 }



 public void OnStateChanged(bool scanning) {

 mIsScanning = scanning;

 if (scanning)

 {
  
 // clear all known trackables

 mImageTracker = mTrackerManager.Instance.GetTracker<ImageTracker>();

  //ImageTracker.TargetFinder.ClearTrackables(false);

   }

  }

  // Here we handle a cloud target recognition event

  public void OnNewSearchResult(TargetFinder.TargetSearchResult targetSearchResult) {

    // do something with the target metadata

    mTargetMetadata = targetSearchResult.MetaData;

     // stop the target finder (i.e. stop scanning the cloud)

     mCloudRecoBehaviour.CloudRecoEnabled = false;

     if (ImageTargetTemplate) {

      // enable the new result with the same ImageTargetBehaviour:

       ObjectTracker tracker = mTrackerManager.Instance.GetTracker<ObjectTracker>();

      ImageTargetBehaviour imageTargetBehaviour =

     (ImageTargetBehaviour)tracker.TargetFinder.EnableTracking(

      targetSearchResult, ImageTargetTemplate.gameObject);

       }

      }

      void OnGUI() {

      // Display current 'scanning' status

       GUI.Box (new Rect (100, 100, 200, 50), mIsScanning ? "Scanning" : "Not scanning");

       // Display metadata of latest detected cloud-target

       GUI.Box (new Rect (100, 200, 200, 50), "Metadata: " + mTargetMetadata);

        // If not scanning, show button

        // so that user can restart cloud scanning

       if (!mIsScanning) {

        if (GUI.Button (new Rect (100, 300, 200, 50), "Restart Scanning")) {

        // Restart TargetFinder

        mCloudRecoBehaviour.CloudRecoEnabled = true;

        }

         }

         }

        }