Q: Payment method decline.

We cannot fix your payment problem. We are users like you. 

And why are you asking in the Aperture forum? It does not seem to be a question about photography.I'll ask fr your question to be moved to the Mac AppSTore forum.

What kind of payment method are you trying to use? A debit card?

Debit cards are no longer accepted in all countries, see this document:

Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

To see your payment options:

You might be able to use other payment types in your country. To see all accepted payment methods:

Click to open iTunes and sign in to your Account Information page, or use these steps:

1. Open iTunes.
2. If you’re not signed in, sign in with your Apple ID.
3. From the menu bar at the top of your computer screen or at the top of the iTunes window, choose Account > View My Account.
   
   If you’re using Windows and can’t see the menu bar, learn how to turn it on.
4. Enter your password, then press the Return or Enter key on your keyboard, or click View Account.

On the Account Information page:

1. To the right of Payment Type, click Edit.
2. On the Edit Payment Information page, you'll see the accepted payment types.
   
   If you don't have an iTunes account, you will see these accepted payment options when you set up a new account.

In countries where the iTunes Store only sells apps, the accepted payment methods are Visa, MasterCard, and American Express. Other payment types such as gift cards, store credit, and PayPal aren't accepted. Depending on your App Store country, prices might be listed in your local currency, US Dollars, or Euros.

²Age varies by country or region.

A common reason might be, that you are not in a country, where your card is valid or the bank declined the payment.

Ceck with the your bank, if they know what happened.

If all fails, ask iTunes Support, if they know the reason.

: http://www.apple.com/support/itunes/contact/