1. Ensure you give some time for DNS to propagate on any new sub domains created.
2. Go to the "Security" tab.
3. Click on the 3 dots at the end of the domain you want to install the ssl certificate to.
4. Click "Request Let's Encrypt certificate".
5. Wait for the process to complete.

NOTE: If it doesn't work the first time, DNS may need more time to propagate. Please allow 30mins before trying again to ensure you don't get blocked by Let's Encrypt for requesting too many times.