I’ve seen this error pop up from time to time when adding a subdomain, addon, or parked domain. This happens when the domain was on the server previously, but was not properly removed.
To correct this, try the following.
1. Remove the new domain name from the following files
2. Remove the following file
3. Remove the virtualhost for the new domain name
4. Run this command once you did the above
5. Remove the newdomain.com cache file
If you try it now, you should be able to add the new domain