i can not purchase Gems in Clash of clans, Please tell me know ?

What happens when you try to buy them, does the 'buy' button not work, do you get any error messages ... ?

If you are getting a message to contact iTunes Support then you can do so via this link and ask them for help (we are fellow users on these forums, we won't know why it's appearing) : 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 ... ?

Note:

Some In-App Purchases require a Credit Card.

To Contact iTunes Customer Service and request assistance

Use this Link > Apple Support iTunes Store Contact