Have you tried a bunch of times and done a bunch of rebuilds and maybe have DNS not pointing to your server? My suspicion is that you’ve requested a certificate several times and it’s failed due to a DNS or intermediate proxy error and now you’ll need to wait a week. You can try another subdomain or add a second address to the cert (it doesn’t matter what it is) as described in Set up Let’s Encrypt with multiple domains / redirects.
Did you run ./discourse-setup? Did it pass the DNS test at the beginning?