Error CS0246: The type or namespace name 'SteamVR_TrackedObject' could not be found. Are you missing an assembly reference?

UNITY VER: 5.6.3p1 VRCSDK-2018.01.26.13.13_Public

Have been working on a world creation and haven't had any problems until recently. I had some 'Amazon' script errors as well but those were fixed once I updated to the most recent SDK. Although this error is still occuring. I believe this is why my VRCSDK drop down option is still only shows "Clear Cache and PlayerPrefs. So I am currently unable to update my world.

I have been looking up if anyone else has similar errors, but nothing has fixed my errors. I have no experience in coding so I don't want to mess with that unless I know exact step by steps.

     /// <summary>
     /// The current controller this component is attatched to.
     /// </summary>
     SteamVR_TrackedObject CurrentController;
 
     /// <summary>
     /// Optional audio source for playing surface hits
     /// </summary>
     AudioSource CurrentAudio; 
 
     /// <summary>
     /// List of controllers used by VRInteractable, can be accessed via the static method GetControllers
     /// </summary>
     private static List<SteamVR_TrackedObject> ControllerList = new List<SteamVR_TrackedObject>();
 
 
     /// <summary>
     /// Gets the controllers currently managed by grippers
     /// </summary>
     /// <returns>List of SteamVR_TrackedObject </returns>
     public static List<SteamVR_TrackedObject> GetControllers()
     {
         return new List<SteamVR_TrackedObject> (ControllerList);
     }
 
     void OnEnable()
     {
         // Cache local components
         CurrentController = GetComponent<SteamVR_TrackedObject> ();
         CurrentAudio = GetComponent<AudioSource> ();
 
         // Add this controller to the static list
         if (ControllerList.Contains (CurrentController) == false)
             ControllerList.Add (CurrentController);