And why are you unable to create a subdomain? Are you missing a button for it ("Add Subdomain") or does the function fail with an error message? Or ist the subdomain configured, but cannot be reached from the Internet?
I have a similar problem. After setting up a Custom Service Plan for my customer I was unable to add a new subdomain or new domain alias (I choose to allow 5 subdomain and 5 domain aliases) but ADD button does not show up in customer's panel.
If I log on with superadmin account I can (obviously) add a new subdomain, but the whole thing is meant to leave some administration to end user.
Subdomains can point to different sections of your website. In order to create a subdomain in your cPanel account follow the instruction below visit this link