If you are getting the error while adding domain (parked or addon)

Error : "Error from park wrapper: Sorry, cannot determine nameserver IPs. Please make sure that the domain is registered with a valid domain registrar."

Log into WHM >> Main >> Server Configuration >> Tweak Settings >> (Enable) Allow Creation of Parked/Addon Domains that are not registered >> Save

Then you would be able to add the domain from cpanel.

This happens because the domain which you are trying to add as parked or addon might not be registered or it might be pointing to another server.