Dmitri Sotnikov @dmitri and Scot Brown @svmbrown
Working through this now and encountered same issue, so thanks for this thread!
In addition, I found on p. 9
A common beginner error that you may encounter is forgetting to start the
database connection. If you do get this error, don’t worry—just run (start) and try again. In fact, any error that mentions mount.core.DerefableState is probably due to forgetting to run (start).
But I agree. There should be an additional sentence telling the reader to not only start the REPL (which it does) but to also ‘(start)’ especially since we are a long way from p. 9!