I see you're trying to update the payment method you use for iTunes & App Store since it's being requested to purchase/update/download apps. When you add your payment method, you're met with a message that it's been declined. I'm happy to help.
If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store -- If there was more to the message than just "declined" review this support article.
Payment methods that you can use in the iTunes Store, App Store, and iBooks Store -- Make sure the payment method you're choosing to use is supported.
iTunes -- Official Apple Support -- If you're still having problems updating your payment information, contact iTunes Support for help. Click the link and choose your country or region, then follow the prompts to contact iTunes Support.