Does the site work WITHOUT SSL?
I have Discourse on an Azure Ubuntu VM and it worked like a charm with no hassles.
First set it up WITHOUT Lets Encrypt. Make sure that you can access the site over straight HTTP.
Then modify app.yaml to include Lets Encrypt and rebuild.
From your error messages, it looks like you have not opened the EndPoint to the public IP address. That’s why Lets Encrypt is not finding the public IP.