The issue is usually caused by an internal iOS setting that you’ve probably enabled a while ago in order to prevent your kids from depleting your credit card. All that you have to do is open the Settings app and follow the steps described below to lift this limitation.
How To Enable In-App Purchases On iPhone and iPad
1. From the Home Screen open the Settings app.
2. Scroll for the new iOS 12 Screen Time section and tap it.
3. Select Content & Privacy Restrictions. If you use a Password for Screen Time you will be prompted to insert it.
Tip: You don’t remember your Screen Time passcode? Tap here to learn how to reset it!
4. Next, tap on iTunes & App Store Purchases.
5. Check the In-app Purchases section. The option is most likely set as “Don’t Allow”. Tap to open the setting.
6. Select, Allow to enable the in-app purchases for your iPhone or iPad.
Tip: If you allow your kids to play with your device, don’t forget to re-activate the restrictions once you finished from purchasing your desired updates! Follow the same steps described above to disable in-app purchases!