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: /home/konw1711/public_html/whitelablehub.id/whitelabel-hub/vendor/midtrans/midtrans-php/examples/notification-handler.php<br><br>Config::$serverKey = '&lt;your server key&gt;';