For those who only administer Discourse occasionally, or newbies, or in general to save time, it’s most helpful to be as specific as possible.
In this particular case, where exactly is the app.yml file? I don’t see one in /var/discourse, but I do see an app-sparkpost.yml. Maybe that’s the file I created ~5 years ago when I deployed the instance; I don’t know. Is that the right path? How does discourse knows to read this file if there’s no app.yml in /var/discourse?