In order to generate Let's Encrypt certificates for domains on your Symb Stretch box the domain in question would need to resolve first to the server; otherwise it would not satisfy the challenges for domain ownership when running the symbiosis-ssl command..
Once the domain name resolves then you can use the following commands:
sudo symbiosis-ssl --verbose my-brilliant-site.com
sudo symbiosis-httpd-configure --verbose my-brilliant-site.com
Symbiosis runs a daily cron to pick up ssl. Any issues please let us know and we can certainly look at this for you.