Class 'PayPal\Rest\ApiContext' not found

I'm having a little problem with my php code, since I'm using paypal api for e-payement.

I get the following error message :

Fatal error: Class 'PayPal\Rest\ApiContext' not found in F:\wamp64\www\tests\form4\start.php on line 7

<?php
require '/vendor/autoload.php'; 
define('SITE_URL', 'paypal4.php');
$paypal = new \PayPal\Rest\ApiContext(new 
\PayPal\Auth\OAuthTokenCredential('ID','ID'));

?>

Any help please ?