Do you mean a "Your purchase could not be completed" message?
You may see various forms of a "Your purchase could not be completed" message. Some may show old URLs (such as the one ending in "/ww/") which will redirect to the current one which leads you to a page to select your country but provides little additional guidance. Since it says to contact iTunes Support, do this:
1. Click on this link --> https://support.apple.com/choose-country-region/media-services
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.