If your payment method/ card is declined probably your bank doesn't let you make purchases on iTunes store. If you were already using ITS and the problem just appeared new, probably you have bad credit. It means you purchased something and the amount couldn't be charged from your card but you got your purchase. So you have debt to ITS. Upgrade you card limit and try to make purchase, when you try to make purchase, system will direct you to edit your payment informations and when you edit it, system will charge the amount and you will be able to purchase now.
If it is the first time that you are trying to purchase then contact with your bank to enable your card for ITS.
If you try to identify more then one card and you got the same error, probably you have a problem with your Apple ID.