I already changing my payment informion and everything fine but i still cannot bay a gems from clash of clans

What happens when you try to buy them, does the 'buy' button not work, if you are getting an error message then what does it say ?

If you are getting a message to contact iTunes Support then you will need to do so (these are user-to-user forums) :

- go to http://www.apple.com/support/itunes/ww

- click on your country's flag

- click on the Contact Support at the bottom of the left-hand column

- click on Contact iTunes Store Support on the right-hand side of the page

- then Purchases, Billing & Redemption

If it's a different problem ... ?