hi.
The your webhook returns status code 500 by also access from my web browser.(Internal Server Error) So the message shown by clicking the verify button is correct. Your webhook must return status code 200, so that getting better message. But this message dosen't mean the Line could not connect your webhook. (It only means the your webhook returns 500) why don't you click the verify button and check the messages after let your webhook simply return http status code 200?