این توضیحی هست که در داکیومنت های خود سایت whmcs ذکر شده:

Server Command Error - Curl Error - Couldn't connect to host (7)

Getting this error means that you have a firewall on your server blocking the connection. The port being used depends upon the module in use but for cPanel it would be 2086/2087, Plesk 8443, DirectAdmin 2222, etc... open for inbound and outbound connections on both the server WHMCS is installed on and the remote server being connected to. Please contact your server admin/hosting provider to open those ports.