Notification-handler are not meant to be opened via browser / GET HTTP method. It is used to handle Midtrans HTTP POST notification / webhook.<code><h4>Please set your server key from sandbox</h4>In file: /var/www/html/lms/vendor/midtrans/midtrans-php/examples/notification-handler.php<br><br>Config::$serverKey = '&lt;your server key&gt;';