Notification-handler are not meant to be opened via browser / GET HTTP method. It is used to handle Midtrans HTTP POST notification / webhook.

Please set your server key from sandbox
In file: /home/digihostmaya/public_html/app-config.digihostmaya.com/paymentOnline/midtrans-php-master/examples/notification-handler.php

Config::$serverKey = '<your server key>';