Initially, windows installation was done by windows CV or DVD, the DVD usually comes with all the necessary keys. Over the time, things get easier and now there are many list of GVLK keys available on internet absolutely free of cost. You need to search and follow the instructions to install those keys.
Your response showed a kms client, which means you ran the command on a kms client or the client key is installed on the server. The client key for Server 2012R2 Datacenter is W3GGN-FT8W3-Y4M27-J84CP-Q3VJ9. Install this on your VMs. Ensure the KMS key you get from MSVL is installed on the KMS server and it is a Server 2012R2 key. For 2012R2 KMS to start activating you will need 5 2012R2 servers or 25 Windows 8 or 8.1 computers to attempt to activate. Once the count is greater than 5 servers will activate and regular windows will start activating after 25.
We have attempted to install that key directly on the client itself and we get the message that "This key didn't work please check it and try again, or try a different key." This key is installed on our server as well. So running slmgr /upk...success. slmgr /ipk W3GGN-FT8W3-Y4M27-J84CP-Q3VJ9 is a success. after this is done then I'm running slmgr /ato and I get the access denied error when running from an elevated Powershell windows as well as cmd prompt.
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