I was struggling the other day with activating Windows Server 2012 R2. It kept saying the key was invalid, I know it was correct. I ended up calling Microsoft and this is what they had me do.
If you are having difficulty activating Windows Server 2012 R2 running on a Virtual Machine, you can trigger a manual activation by doing the following:
Launch Command Prompt with Admin rights.
Run the command slui 4
This will launch the phone activation service.
Little did I know after I activated it successfully, it would not join the domain… After doing some more digging, it appears I set up the virtual switch in Hyper-V to point at one of the four Network Interface Cards (NICs) that was not connected. Man, I felt dumb after that!