Got this error when trying to add addon domains in cPanel accounts. This happens when cPanel doesn’t remove the addon/park domain correctly and then it thinks it’s still there so when you try to add it back on you get an error.

Here’s how to fix it:

1 .Check if there is any park domain entry available for the domain name in question
2 Remove from /var/cpanel/users/cpanel-username
4. Run /scripts/updateuserdomains as root user on the server because you changed the above file manually, this will create adjusted cache files
5. Remove /var/named/ if the file exists (it doesn’t always)
6. Remove the virtualhost for on /usr/local/apache/conf/httpd.conf
7. Remove from /etc/named.conf

Now you can add the domain back on in cPanel with no problems.
Try to add addon domain again, this should be resolve your issue.