If you just setup your Subdomain it's possible the SSL certificate hasn't automatically installed. For this to happen, the DNS records must be propogated so the Let's Encrypt module can verify domain identification.
If you'd like to manually try to run it, you can follow these steps:
- Login to cPanel
- Click SSL/TLS Status
- Check the box next to the red beside the subdomain (as noted in the screenshot above)
- Click Run AutoSSL
This will attempt to force run the AutoSSL module. It takes a few minutes to run, so just set and let it do its thing. If it fails, it's likely you need to wait another hour or two for the DNS to propagate. This process should be automated if waiting 24 hours or more.