If your plan allows for sub domains you must have them manually enabled. You need to log into cPanel first. 

And then

1. Click on the Manage SubDomains link in the Domain area. 
Please Note: You can only successfully create a new subdomain once your DNS number has successfully propagated for this account. 

2. Enter the text for the new subdomain in the field in the available field. You must conform to domain name conventions for the addition to the name. 

3. Click on Add the button. Click on the Sub Domains link to return to the Subdomains window to confirm that it has been added. A folder with the same name as the subdomain has now been added to your public_html folder, with a cgi-bin sub-folder (if CGI scripts are enabled on your account).