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 mentione