Will a
./launcher start app
get the forum running?
My best guess is that you inserted a tab or something else that makes the yml invalid.
The error is about the “echo” at the very end of the yml file, which suggests that there’s a formatting issue somewhere before that.
Did you save a copy of the file before you edited it, or have you attempted to edit it to restore it to what you think its state was before you edited it?
A thing you can try is to rename your app.yml, take note of the values (especially the SMTP stuff) and run ./discourse-setup again. That’ll give you a working app.yml.