Error "Product key not found" or "Product not found" when Activating Windows with KMS

Symptom

slmgr /ato with error: Product not found.

slmgr /dlv with error: Product key not found.

Resolution

Install License Key

Get KMS client setup keys from Microsoft.
Here are some windows server KMS client setup keys:

Windows Server 2008 R2 Enterprise: 489J6-VHDMP-X63PK-3K798-CPX3Y
Windows Server 2012 R2 Standard: D2N9P-3P6X9-2R39C-7RTCD-MDVJX
Windows Server 2016 Standard: WC2BQ-8NRM3-FDDYY-2BFGV-KHKQY

Install setup key with slmgr /ipk XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

Point to your KMS server

slmgr /skms XXX.XXX.XXX.XXX

Active windows

slmgr /ato