Q: I have a valid debit card that keeps on getting declined. I called my bank and card is ok.

Did you report unauthorized purchases on your account to get a refund?  If so, you will need to contact iTunes store support'

Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

Im having the same issue. I changed my debit card and bam , can't make any in app purchases. i even tried a different credit card and nada. Why is so difficult to resolve this? We can't call someone about this? so how does this work????????

Please contact iTunes Chat Support. They will be able to rectify this issue.

https://www.apple.com/in/support/itunes/contact/