If you're seeing an error message about in-app purchases not being allowed when trying to upgrade to premium, don't worry! There's a very simple and quick fix:
Open the Settings screen on your iOS device
Finally, tap the toggle to enable In-App Purchases
(Note: you may be required to enter your device password at some point during this process)
Why was this off to begin with?
If you have children or are sharing a device with a family member, in-app purchase permissions may have been restricted to prevent accidental purchases. Once you've completed your in-app purchase with Pocket Prep, you can follow the above steps to re-enable the restrictions on in-app purchases.