Sometimes the Activate now button in Control Panel -> System doesn’t work, or throws a vague error message. In this case, it might be worth a try activating Windows from the command line instead.
Start a command prompt as Administrator
Add the new license key
slmgr.vbs /ipk your-key-here
Activate Windows using the previously added key
If something goes wrong, the slmgr script will show a detailed error message, unlike the GUI which just throws a generic error message.