So, webpacker is not generating the application.css file for some reason – this is a webpack error that sometimes happens intermittently when webpacker gets into a strange state.
Are you running this off the sample code directly, or have you been trying to build the app yourself? Is there another error in the logs that suggests that the app/packs/entrypoint/application.scss file didn’t compile?
You could try removing the /public/packs directory and trying again?
I’m sorry you are having this problem.
Alternately, the second edition of the book doesn’t use webpack, and should be easier to set up, if you purchased the ebook from Pragmatic, you get the second edition for free, and it might be worth you trying out.
Noel