"Serialization of 'PDO' is not allowed" Cache Error

When i run this code, it gives above error

MainCategoryController.php

  public function index(Request $request)
    {
        $currentUserRole = Auth::user()->roles->pluck('name')->first();

        if ($request->ajax()) {


            if ($currentUserRole === "student") {
                $userId = Auth::id();

                //Get Assigned Users Courses
                $userAssignedCourses = User::with(['courses' => function ($query) {
                    $query->with(['subCategory', 'mainCategory']);
                }])->findOrFail($userId);

                $userAssignedCourses = $userAssignedCourses->courses;
                $data = $userAssignedCourses->pluck('mainCategory');


            } else {

                //  $data = MainCategory::orderBy('id', 'desc');


                $data = Cache::put('data', MainCategory::orderBy('id', 'desc'),config('cache.expire_time.short'));


            }

          .....
        }

    }