Payments are most often rejected on Google Play for the following reasons:
1. using a virtual credit card (such cards are currently not accepted by Google Play),
2. expired card information (in particular, the expiry date),
3. incorrect card information (in particular, incorrectly entered number),
4. incorrect security code (CVC).
5. billing information inconsistent with the information provided by your bank.
More information on rejected payments can be found on the Google Support Centre