Should you encounter difficulties during the purchase of any of our apps on the App Store or Mac App Store, Apple has provided detailed guidance on this matter:
If your payment method is declined in the App Store or iTunes Store
For our European customers, there may be a need to verify credit or debit card information with their card issuer. For further insights, refer to Apple's article on billing and purchases in Europe:
About billing and purchases in Europe
These resources should provide the necessary assistance.