Looks like ./discourse-setup can’t handle that.
So you will need to edit the app.yml.
cd /var/discourse
nano containers/app.yml
Remove the # in the start of the lines that mention one of the following (they should be at the top of the file)
- web.ssl.template.yml
- letsencrypt.ssl.template.yml
Also remove the # in the line that mentions Let’s Encrypt email, and put a valid email in the front.
After that running a ./launcher rebuild app should do it.