If you are wanting to activate your windows 2012 server, perhaps you need to change the installed product key there is real fast and easy way to accomplish the task.
Changing installed key
Open elevated command prompt PS or CMD on the server and type below command
Uninstalled product key successfully
Now lets install the correct product key
|slmgr –ipk XXXXX-YYYYY-ZZZZZ-OOOOO-LLLLL|
you are done