Laravel 8 - Class not found error using SDK

I'm using the SDK provided online to connect the payment API, and I'm sure the SDK can work successfully. However, as I followed the steps setting up my code, I got an error. I think it might because of the different Laravel version I used.

I want to use the class "Newebpay" in my PurchasesController.php, but I got the error message: Class 'Newebpay' not found.

Here is my PurchasesController.php code:

<?php

namespace App\Http\Controllers;

use Illuminate\Foundation\Auth\Access\AuthorizesRequests;
use Illuminate\Foundation\Bus\DispatchesJobs;
use Illuminate\Foundation\Validation\ValidatesRequests;
use Illuminate\Routing\Controller as BaseController;
use Newebpay;

class PurchasesController extends Controller
{
    use AuthorizesRequests, DispatchesJobs, ValidatesRequests;

    public function index()
    {
        return view('purchases.index');
    }

    public function purchase()
    {
        $newebpay = new NewebPay();
        return $newebpay->payment(
            '549881412984', // 訂單編號
            123, // 交易金額
            'test', // 交易描述
            'kk@gmail.com' // 付款人信箱
        )->submit();
    }
}

I've followed the instruction in the SDK, adding 'NewebPay' => Treerful\NewebPay\NewebPay::class,in the \config\app.php 'aliases' part as the code below.

'aliases' => [

        'App' => Illuminate\Support\Facades\App::class,
        ...
        
        'NewebPay' => Treerful\NewebPay\NewebPay::class,
    ],

For more details, here is my git repository of this project. https://github.com/shigenogoro/testing_laravel