I had this error when I was going through the book using Rails 6.1.
The problem is that the steps in the book don’t allow webpacker to be installed, and you’re seeing an error because its config file isn’t present.
I posted a topic about what I had to change to use Rails 6.1 here. It will fix your problem if you’re using Rails 6.1, and may fix it even if you’re using an older version of Rails.