I can't make in-app purchases with this phone

What happens when you try? Error message(s)?

See the Apple Support article(s) below. 

Apps won’t downloaded.  *

ASC YouTube for what to do if you can’t make a purchase on your iPhone -

App Store, iTunes Store, or other Apple services - If you can't connect   *

If necessary, contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide   -

Apple Support schedule phone call     -

Billings and Subscriptions   -

Apple on Twitter