Q: Why I cannot purchase coin in my another device for game?

We are fellow users here on these user-to-user forums, if you would like help then you will need to provide info as to what happens when you try to buy them e.g. does the 'buy' button not work ? Are 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 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 ... ?