What version of phoenix are you using? The current B8 of the book I think is based on phoenix 1.5 and LiveView < 0.18. As a result, there may be a lot of code that is incorrect if you’re using Phoenix 1.7 (the latest release).
@SophieDeBenedetto has mentioned on a few threads that they are just about to release B9 of the book which is being updated for the current versions of Phoenix & LiveView. 
At this point you may want to wait on the book until B9 has dropped.