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: