When I try to add a domain, I receive the error A DNS entry for the domain DOMAIN_NAME already exists. What can I do?

This error message refers to the fact that there is already a DNS record in the Hostico DNS cluster for the domain you are trying to add.
More specifically, the domain is already added to a Hostico shared hosting account (either yours or another client's)
If the domain exists on a personal account, it will need to be deleted in order to add it to the new location (site migration may also be required)
If you cannot find the domain, the technical department will need to be contacted for further investigations.