In some cases this error you're seeing relates to a temporary network issue that should resolve after some time has passed.
However, if you find it persists, this seems to be a common issue for apps that use the iTunes Store for in-app purchases. Please refer to this page for further information.
The solution most users have found is to sign out of your iTunes account in the Settings -> Store menu, and then try the purchase in-game - you should then be prompted to sign in from there, and that seems to work for most users.
If none of those solutions work, it could be an account-side issue, in which case you might want to send a request to Apple's customer support team.