When the server operating system is installed using the retail or volume licensing media, the operating system license must be activated. On the Server Core operating system, activate the license by using either the sconfig tool or the slmgr command.
To activate the operating system license by using slmgr, see https://technet.microsoft.com/library/dn502540(v=ws.11).aspx.
To activate the operating system license by using the sconfig command, see https://technet.microsoft.com/windows-server-docs/get-started/sconfig-on-ws2016.
For volume activation of the Windows operating system in the data center, see the Microsoft documentation for using KMS.