Purchase problems occur for a few different reasons, their fixes depending on device type.
Apple:
Because Apple processes payments made toward the game, it’s important to have your device settings properly configured before making a purchase.
If you’ve made an order that has been processing for 24 hours or more, please contact iTunes Support here.
If you are not waiting on a purchase, please check the iTunes in-app purchase restriction guide for further information.
Android:
If you are trying to make a purchase with your Google account, it’s possible for you to experience the issues mentioned above. In order to resolve these, please follow the link to Google Play Support.