You'll be prompted for a password, or Touch/Face ID for any monetary transaction in the App Store or iTunes store. You can either set up a password protection or enable Touch/Face ID.
Option 1. Turn on Touch/Face ID for iTunes & App Store purchases if you don't have it enabled:
- Settings → 'Touch ID & Passcode' or 'Face ID & Passcode' → Use Touch/Face ID for: iTunes & App Store (toggle on)
Option 2. You can turn off Touch/Face ID for iTunes & App Store purchases and turn on password protection.
NOTE: it's not possible to have both Touch/Face ID and password protection at the same time.
- To turn off Touch/Face ID for iTunes & App Store purchases: Settings → 'Touch ID & Passcode' or 'Face ID & Passcode' → Use Touch/Face ID for: iTunes & App Store (toggle off);
- To turn on password protection: Settings → Your name (Apple ID, iCloud, iTunes & App Store) → Media & Purchases → Password Settings → Choose a password protection option (Always Require/Require After 15 Minutes).
Option 3. Alternatively, you can disable in-app purchases entirely:
- iOS 12 and higher:
Settings → Screen time → Content & Privacy Restrictions → iTunes & App Store purchases → In-app Purchases
- iOS 11 and lower:
Settings → General → Restrictions → In-app Purchases
To learn more about Touch ID/Face ID or password preferences read this article: https://support.apple.com/en-us/HT204030