There is an error in Cpanel hosting server that cause Add-on domain is not working properly.

Here is a various symptoms:
* When user adding add-on domain and delete it and then re-adding it again, it will show error
* Or if the domain to be added was a single user account on the same server (even after being terminated) it cant be added
* Add-on just disappear after CPANEL update

The fastest solution is :
Login into your Cpanel server with root access.

We will have an example domain name : testing.com that have a problem with add-on domain.

Look at this files or directory and cleanup or fix :
* File /var/cpanel/users   (find : testing.com user and delete it)
* Directory /etc/valiases (find : testing.com and delete it)
* Directory /etc/vdomainaliases (find : testing.com and delete it)
* Directory /etc/vfilters (find :testing.com and delete it)
* File /etc/named.conf ( find : testing.com and delete it)
* Directory /var/named (find : testing.com.db and delete it)
* File /etc/httpd/conf/httpd.conf (find : testing.com and delete it)

After we've clean up, we need to make sure restarted the HTTP and BIND services.

Restarting Services

Login into your Cpanel

This should fixed the Cpanel Addon domain problem