Manage In App Purchases. Go to settings > general > restrictions. You may need to sign in with your apple id.
Manage your iTunes Store and App Store password preferences Apple Support
Enter your password, or authenticate using touch id or face id, if prompted. Sing into the itunesconnect choose my apps category open an app you want to create inapp products for (or create an app, if you haven't done it before) add products for your application under the features tab as shown on the screenshot bellow The most important of these are:
Manage your iTunes Store and App Store password preferences Apple Support
On the list of apps pane, choose the app you want to assign, and then choose properties. You can also add localizations to ensure that people in regions where your app is available have a seamless purchase experience in their preferred language. Enter your password, or authenticate using touch id or face id, if prompted. Sing into the itunesconnect choose my apps category open an app you want to create inapp products for (or create an app, if you haven't done it before) add products for your application under the features tab as shown on the screenshot bellow