There are several possible causes for that. Please follow these steps to resolve the issue:
Check to see if your credit card information is up to date in your mobile App Store account. Charges are managed directly with your mobile App Store and even if you have credit available with them, you may need to have this updated.
Some App Stores require you to accept new Terms & Conditions, even when users have funds available. Make sure you have updated the latest Terms & Conditions with your mobile App Store account.
Make sure you connect to a reliable Internet source (a steady connection with enough bars).
All purchase transactions in the game are handled directly through your App Store. In order to change your payment information, you will need to make this change in your App Store settings. This can be done either directly on your device or through the service provider's website. Here are the links for different platforms’ service providers:
Apple (iOS) support:
Google Play Store (Android) support: