You need to serve that app to use on your trello boards, for that there is a node script server.js on the project root you could run that using:
npm run start
That will only work for local tests to use on trello you need to use a host like AWS, digital ocean, heroku, etc.
After that you need to access that page to enable custom power up on trello:
https://trello.com/power-ups/admin
More about that process here https://tech.trello.com/power-up-tutorial-part-one/
On the popup you must provide your information about github token, the list to create pull requests cards and the github project after that you click on the save and on the sync pull requests button to create new cards. easy like that
