Hmm, not sure what that's about. Do you have another server that's connected to localhost:3000? Although if you do, you wouldn't have been able to start up the server.
Could you drop a screenshot of the browser on localhost:3000 and maybe a snippet of the server logs?
oh wait, there we go! I think I did have something else running on 3000—not surprising, since I am always running like five development servers, but I should have double-checked that port was open. Thanks!
OH I think I know what it is. You might have went to localhost:3035 which is running Webpack. It might not be exactly 3035, but it's something similar and is not localhost:3000. localhost:3000 is the port for the backend Rails server, which is the one you want to visit.
Also, now that I've got it set up, props to whoever on your team likes this book:
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