At times while adding the domains eg add on domains, alias domain, you may encounter an error as such:
A DNS entry for "domain.com" already exists. You must remove this DNS entry from all servers in the DNS cluster to proceed.
Replace "domain.com" with the domain name that you are trying to add.
If you facing above error when you try to addon domain to your cPanel, it probably means that the domain already exist in the same cluster, hence it is not able to addon the domain.
Do contact support team so that we could provide further assistance.