Some times when you getting an error while create subdomain in cPanel (WHM)  The system experienced a problem during the creation of the "test" subdomain. Assume that the subdomain name is here "test".

From the Log:

[2015-06-02 10:08:37 +0000] warn [SubDomain::addsubdomain] Encountered error in SubDomain::addsubdomain: [A fatal error or timeout occurred while processing this directive.]

You need to check the following tweak settings,

 Tweak Settings -> Allow users to park subdomains of the server’s hostname.
    Off to On

Another solution, may be the error on Server Time Zone, So must be check your server time. or kindly contact to your cPanel Support.