Could not determine the nameserver IP addresses for “domainname”. Please make sure that the domain is registered with a valid domain registrar.”
Client will need to ensure the domain name is registered and uses name servers associated with your server before adding it as a parked or addon domain name.
Go to WHM >> Server Configuration >> Tweak Settings >> All
Check for the following option “Allow unregistered domains” it might be “Off” on your settings. Change it to “On” if you want to add unregistered domains as Addons.
2) From command line:
Edit the cpanel config file.