I want to buy gems in app clash of clans but they ask contect the apple support

Hi

Follow the instructions here as appropriate, including checking that your address details fully match those on file with your card issuer / bank:

• Pay an unpaid balance in the iTunes Store - Apple Support
• Change or remove your Apple ID payment information - Apple Support
• Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

If your payment method is a bank card, you may also wish to check with your bank, in case it has declined Apple's payment authorisation requests.

If you still need help, contact iTunes Support:

Contact - Official Apple Support