Why Payhere payment notification not receiving

I have tried to use sandbox payment using moodle and payment is successful and shown in payhere dashboard. but in moodle, the student is not enrolling and again show a payment option. [![error image][1]][1] [1]: https://i.sstatic.net/WIkXp.png but from payhere side, the call for notify is not receiving to proceed with enrollment but after send details with postman it work perfectly.