Pusher is a third party service being used to power the chat system.
In order to use the chat functionality within you development environment you will need to sign up for a free-tier Pusher account and retrieve its keys. Then you'll need to provide those keys to the Rails application.
Once signed in, fill in the prompt to create a new Pusher Channels app.
In your new Pusher Channels app, click the "App Keys" tab.
Change your keys accordingly (name of Pusher key -> name of our application key):
1 2 3 4
app_id -> PUSHER_APP_ID key -> PUSHER_KEY secret -> PUSHER_SECRET cluster -> PUSHER_CLUSTER