Use the document Add a payment method to your Apple ID - Apple Support to review your payment methods such as credit or debit cards. Problem methods will usually have a notice in red lettering below them. The article If your payment method is declined in the App Store or iTunes Store - Apple Support has some information about declined payment methods.
In the meantime, try paying for the item with account balance from gift cards. How to redeem your Apple Gift Card or App Store & iTunes gift card --> How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support
Providing a comprehensive listing of all possible issues and their solutions would be excessive in a single reply. If the information above does not indicate the nature of the problem then you may find the fastest route to resolution would be to contact Apple for assistance with this.
To contact iTunes Support, do this:
1. Click on this link --> Choose your country or region - Official Apple Support
2. Select your country
3. Select: Billing & Subscriptions
4. Select: Subscriptions & Purchases
5. Select: Update payment method
6. Select a chat session or a phone call.