Why i can't purchase the apps gem in the apps /?

We are fellow users here on these forums, you're not talking to iTunes Support nor Apple, if you would like help then you will need to provide more information e.g. what happens when you try to buy them, if you are getting an error message then what does its say ?

If you are getting a message to contact iTunes Support then you can do so via this link and ask them why the message is appearing (we won't know why) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

If it's a different problem ... ?