Your app is failing to start as it did not bind to the correct port.
Scala apps on Heroku need to bind to the value given by the
PORT environment variable. This is set to a randomly chosen port each time the dyno boots up.
To read this when running on Heroku, but fall back to a known port when running locally you can use the following:
val port = Properties.envOrElse("PORT", "8080").toInt