In order to use the chat functionality within your 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):
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app_id -> PUSHER_APP_ID key -> PUSHER_KEY secret -> PUSHER_SECRET cluster -> PUSHER_CLUSTER