Have you seen the following error when attempting to add an addon domain or subdomain to cPanel?
This is a common issue that occurs when there is a pre-existing DNS record or domain. To resolve this issue, check the DNS zone of the corresponding primary domain and see if there is a DNS record already added for the addon domain or subdomain that you are trying to add.
If a DNS record exists, you cannot add the new domain until you remove the record and the corresponding entries in DNS zone.
If a DNS record does NOT exists, make sure there is not a subdomain present and related to the same name in your cPanel.
If no subdomain exists, but a DNS entry exists, it's probably a remnant of any previous domain management activity. Simply remove the DNS entry and try adding your new domain again.