I deployed what I created with Spring Boot to Heroku via Github.

It works fine locally, but when I try to check what I have deployed on Heroku by pressing the Open app button
There was an unexpected error (type = Internal Server Error, status = 500).
Appears and the page does not open.

The Controller that calls the top page is described as follows.

@RequestMapping (value = "/", method = RequestMethod.GET)
public String Top (Model model) {
    return "Transition destination";

This is my first time using Heroku, and I'm in trouble because I don't know right or left.
I would appreciate it if you could teach me.