Open TCP port 443 in the firewall to enable vCenter Server to receive data. By default vCenter Server uses TCP port 443 to listen for data from its clients. If you have a firewall between vCenter Server and its clients, you must configure a connection through which vCenter Server can receive data from the clients.
Firewall configuration depends on what is used at your site, ask your local firewall system administrator for information. How you open ports depends on whether you use a vCenter Server Appliance or a vCenter Server Windows installation.