Q. I've enabled Key Management Service (KMS) on a server, but clients can’t activate. Why not?

A. There’s a minimum number of activations before a KMS server will start activating clients. However, if the clients can’t even connect, make sure the firewalls on the server and client have the Key Management Service (TCP-In) exception enabled, which allows KMS communications through port 1688 as shown.

The error if the firewall exception isn’t enabled is:

C:WindowsSystem32>cscript slmgr.vbs /ato
Microsoft (R) Windows Script Host Version 5.8
Copyright (C) Microsoft Corporation. All rights reserved.

Activating Windows Server(R), ServerEnterprise edition (620e2b3d-09e7-42fd-802a-17a13652fe7a) ...
Error: 0xC004F074 The Software Licensing Service reported that the computer could not be activated. The Key Management Service (KMS) is unavailable