I just purchased a new laptop and did a complete reinstall on the old one so I can give it to my son. Full install of Windows 8 Pro, ran all windows updates and then installed the free upgrade to 8.1 Pro from the store. When I try to add features, it asks for a product key. The windows 8 product key isn't accepted any longer.  What should I do?

Thanks in advance for any help,

What "features" are you trying to install?  Would that be windows media player/center which requires a separate key?

Nothing specific. I can't even look to see what options are available. Could this have anything to do with the store itself?

I just purchased a new laptop and did a complete reinstall on the old one so I can give it to my son. Full install of Windows 8 Pro, ran all windows updates and then installed the free upgrade to 8.1 Pro from the store. When I try to add features, it asks for a product key. The windows 8 product key isn't accepted any longer.  What should I do?

That's how it's always been?

"Add features" is the MSFT way of saying "move to an upgraded edition of Windows", eg Home -> Pro. This requires a product key, which must be purchased. ["Add features" was previously known as "Windows Anytime Upgrade" in Win7/WinVista]

If you have Win8.1Pro, the only upgrade available is to add the "Media Center Pack":
http://windows.microsoft.com/en-au/windows-8/feature-packs

But maybe you are looking for "Turn Windows features on or off", which doesn't use the Store, doesn't require a product key, and doesn't require a purchase.
Use this to enable dormant-by-default features, such as enable games, or enable HyperV, etc.
http://windows.microsoft.com/en-AU/windows-8/add-features-frequently-asked-questions

Thanks Don. Couldn't ask for a better explanation!