Depending on a number of factors, you may be faced with a similar issue.  In a nutshell, the native UI in Windows 8 informs you that it is unable to activate windows right now, please try again later.  After some digging around, I realized that I never actually entered my key during setup, and maybe that was the underlying issue.

Problem is; there is no easy way to input your key in the Windows 8 UI.  But there is a simple enough workaround:

Activating Windows 8

 

  • Open Start Menu.
  • Type "run" and press enter to invoke it.
  • Type "slui 3" and press enter.
  • Enter your product key and Activate.
  • Done!

 

More here:

social.technet.microsoft.com