Q: I'm not able to make in-app purchase.

Have you contacted iTunes Support as the message says to do ? If not will need to do so (these are user-to-user forums) :

- go to http://www.apple.com/support/itunes/ww

- click on your country's flag

- click on the Contact Support at the bottom of the left-hand column

- click on Contact iTunes Store Support on the right-hand side of the page

Or use this form : https://www.apple.com/emea/support/itunes/contact.html