The easiest thing is probably to delete your app.yml and run discourse-setup again. It could literally be a single extra or missing space most anywhere. Or tab instead of some spaces. Maybe you can figure it out with yamlint, or maybe you’ll always mess something up when you copy to or from it.
If what you did to break it was add plugins, see that next time you copy/paste the existing git clone line and edit it. That way you can’t mess up the spacing.