Here are some of the reasons why this might happen:
Your internet connection. It has to be stable in order to make purchases, so we recommend using Wi-Fi or at least a 3G/4G connection while making purchases.
Temporary problems with the app store. All in-game purchases are processed by the store where the game was downloaded from. There is a chance that the store is overloaded and unable to process your request. In that case just wait several minutes and try again.
Problems with the store app. These are usually solved by restarting your device.
If you've never made purchases in the game or other applications:
Incorrect payment information. If you made a mistake while entering your payment information, cancel the payment and start over.
Unsupported payment method. It may be that the payment method you want to use is not supported by the app store. The list of supported payment methods can be found in the respective sections of the app stores' support websites.
All of the purchases are processed by the store from which you have downloaded the game. The solutions mentioned above are based on the most common potential issues. If you are still not able to make a purchase, please contact the store from which you downloaded the game as they will have all of the necessary information to assist you in this case.