Issues preventing players from making in-app purchases are usually related to: incorrect device/security settings, not being logged in to your iTunes or Google Play account, or a poor internet connection.
Please first check to see if your iTunes billing information is up-to-date. For more details, check here:
Furthermore, please make sure that you are logged in to the iTunes app store with your Apple ID account before making a purchase in the game. If this issue persists, please check to see if your settings for in-app purchases are enable. To see if your in-app purchasing option is enabled for your iOS device, please refer to the following page: