vCenter Server uses TCP port 443 to listen for data transfer 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.
Open TCP port 443 in the firewall to enable vCenter Server to receive data from the vSphere Web Client. Firewall configuration depends on what is used at your site, ask your local firewall system administrator for information.
If you do not want to use port 443 as the port for vSphere Web Client-to-vCenter Server communication, you can switch to another port by changing the vCenter Server settings from the vSphere Web Client. See the vCenter Server and Host Management documentation.
If you are still using the vSphere Client, see the vSphere Administration with vSphere Client documentation.