Check this -> Require a password for App Store and iTunes purchases
Require a password for App Store and iTunes purchases - Apple Support
See also the following Apple articles:
Manage App Store purchases, subscriptions, settings, and restrictions on iPhone
Manage App Store purchases, subscriptions, settings, and restrictions on iPhone – Apple Support (UK)
Prevent in-app purchases from the App Store
Prevent in-app purchases from the App Store – Apple Support (UK)
If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.