why i cannot buy in-app product
I tried credit card and debit card, but neither of them work.
iPhone 6 Plus, iOS 8.3
What do you mean by 'cannot buy', what happens when you try to do the IAP, if you are getting an error message then what does it say ?
If you are getting a message to contact iTunes Support then you 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
- then Purchases, Billing & Redemption
If it's a different problem ... ?