Seems you have some lines missing from app.yml if you change your mind after setup and want to use email after all.
I’m not so familiar with the way things have changed since Self-Hosting Discourse Just Got a Whole Lot Easier but on my sites the way SMTP details are configured in app.yml have not changed. if I were you I would try running
cd /var/discourse
./discourse-doctor
and see if that allows you to fix it. If that doesn’t work, then I would go find an app.yml from a working site and use that as an example to make sure the lines required for email are present and correct. If you don’t have that, then you can get an example here:
Then rebuild.
If all of this feels hard, if it’s not too late you can also start over and answer the questions correctly with your SMTP details from the outset.