致命错误：未捕获错误：未找到类'Paypal \ Api \ Payment'

Help guys! getting error as said in the title. Been trying to figure it out but i haven't found any helpful answered as of now. Thank you.

Fatal error: Uncaught Error: Class 'Paypal\Api\Payment' not found in C:\xampp\htdocs\paypal\pay.php:19 Stack trace: #0 {main} thrown in C:\xampp\htdocs\paypal\pay.php on line 19

<?php

use Paypal\Api\Payment;
use Paypal\Api\PaymentExecution;

require 'app/start.php';

if(!isset($_GET['success'], $_GET['paymentId'],$_GET['PayerID'])) {
   die();
 }

if ((bool)$_GET['success'] === false){
   die();
}

$paymentId = $_GET['paymentId'];
$payerId = $_GET['PayerID'];

$payment = Payment::get($paymentId, $paypal);
$execute = new PaymentExecution();
$execute->setPayerId($payerId);

try {
    $result = $payment -> execute($execute, $paypal);
} catch (Exception $e) {
    die($e);
}
?>