Some iPhone apps have additional features or content which you can only access by making in-app purchases.
In general, you don't need to do anything special to enable in-app purchases — just open the app, find the feature or content you want to buy, and complete the in-app purchase according to the instructions in the app. It'll use the payment method connected to your Apple ID account.
If you can't make an in-app purchase on your iPhone, though, it's likely that they have been disabled in the Screen Time settings on your phone. Here's how to enable them again.
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How to enable in-app purchases on iPhone
1. Start the Settings app on your iPhone.
2. Tap "Screen Time."
3. Tap "Content & Privacy Restrictions."
4. If it's not grayed out, tap "iTunes & App Store Purchases." If it is grayed out, then you don't have Screen Time set up and should skip to the next solution.
5. Tap "In-app Purchases."
6. If In-app Purchases is currently set to "Don't Allow," tap "Allow."
7. Tap "Back" and close the Settings app.
You should now be able to make in-app purchases.
If they weren't disabled in Screen Time, though, or if making that change didn't enable in-app purchases for you, there's one other possibility: Your payment information associated with your Apple ID might be out of date.
Here's how to check that:
1. Start the Settings app and tap your name at the top of the screen.
2. Tap "Payment & Shipping." You might need to enter your Apple ID password.
3. Verify that your credit card or PayPal information is up to date. If needed, add or edit your payment information.