I cannot purchase clash of clans

You will need to contact iTunes support and ask them why the message is appearing (these are user-to-user forums, 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 try selecting Purchases, Billing & Redemption

Your in app purchase may be disbaled as well, go into settings>general>resterictions to find out. Also you may not have any credit in your account.