Why me don't buy gems in clash of clans game

What happens when you try to buy them ? If you are getting a message to contact iTunes support then you can do so via this page and ask them why the message is appearing (these are user-to-user forums, we won't know) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page

If it's a different problem ... ?