Error uploading file using dropbox graham (Laravel)

I've got a form and when I click submit and It calls the controller but this gives me an error:

• Class 'GrahamCampbell\Dropbox\Facades\Dropbox\WriteMode' not found

Also I tried to create a Folder in my dropbox to check If there are some problem to call the class but It works.

<?php

namespace App\Http\Controllers;

use GrahamCampbell\Dropbox\Facades\Dropbox;

use Illuminate\Http\Request;

use App\Test;


class ArticulosController extends Controller
{

public function store(){

            $file = fopen('path/test.pdf', 'rb');
            $size = filesize('path/test.pdf');

            Dropbox::uploadFile('test.pdf', Dropbox\WriteMode::add(), $file, $size);
            Dropbox::createFolder('/example');  //This function works

        return redirect ('/home');
    }

}

In spite of I tried to call WriteMode class, for some reason the function doesn't call WriteMode correctly. So I finally use Dropbox Adapter and know it works.