Many of the examples will require third-party systems (to send messages to), which you may or may not be using. If you do not have access to one of these systems, the relative example will not be executable for you. This is not a big problem, since you can still read the section and many notification modules are very similar to another. You might find that a module that is more suitable for your environment is present and functions in a very similar way. For a full list of Ansible notification modules, you can refer to https://docs.ansible.com/ansible/latest/modules/list_of_notification_modules.html.
You can download all of the files from this book's GitHub repository at: https://github.com/PacktPublishing/Learning-Ansible-2.X-Third-Edition/tree/master/Chapter06.