If you get hit with a message on your Apple iPhone or iPad that says “Purchase – In-app purchases are not allowed” when trying to buy purchases from within apps, it may be related to a restriction setting on the device.
- From the Home screen, swipe over to the screen with the “Settings” icon, then select it.
- Select “General“.
- Scroll down and select “Restrictions“.
- Type your restriction password. If you don’t remember setting a password, ask anybody else who may have had access to the device.
- Scroll down to the “In App Purchases” option and ensure it is turned “On“.
You can also check for more details at APPLE's official site: https://support.apple.com/en-us/HT204396