Servers
From WHMCS Documentation
Adding a New Server
Deleting a Server
You cannot delete a server while there are any accounts assigned to it. That includes terminated and cancelled accounts. You should not consider deleting a server that is/has been used. To actually delete it, go to Configuration > Servers and then click the red X icon on the same row as the server you wish to delete. If it is allowed to be deleted you'll get a confirmation popup confirming you want to delete it and once you click Ok it will be removed.