It looks like you haven't actually deployed the code. Once deployed a free Mailgun account is created for you behind the scenes and assigned to your app. To test locally you need to create a Mailgun account and follow instructions from the "Extra optional step" section.
That is because I don't have a web site to deploy to yet. I wanted to test it on my Ubuntu box on Apache first before deploying. Why do I have to create an account on some other web site just so I can send email on my local Ubuntu machine?
It looks like you haven't actually deployed the code. Once deployed a free Mailgun account is created for you behind the scenes and assigned to your app. To test locally you need to create a Mailgun account and follow instructions from the "Extra optional step" section.
BR, Sergey