Reason:

 

You might see this error after adding the Windows Media Center Pack and trying to enter your new product key.

Unable to activate Windows 8.1 Pro with Media Center or Windows 8 Pro with Media Center

 

Resolution:

To fix the problem, you can activate Windows by phone.

 

Swipe in from the right edge of the screen, and then tap Search.
(If you're using a mouse, point to the lower-right corner of the screen, move the mouse pointer up, and then click Search.)

Enter SLUI 04 in the search box, and then tap or click SLUI 04.

Follow the instructions on the screen.