The problem is that your lightsail instance isn’t contactable externally. That’s what discourse-doctor was telling you - if you only opened port 443 this is probably why.
HTTPS on 443 isn’t going to work until you have a certificate. You don’t have a certificate (and those certificate files are empty) because let’s encrypt was unable to enroll due to the communications issue.
Assuming you assigned a static external address also open port 80 then try again. 80 will need to be open anyhow so that any client which connects via HTTP can be redirected.