Q: I can't make in app purchases on iPhone 5s

I brought an iPhone 5s second hand, fully working and factory reset. I've set it all up and I've signed in with my Apple ID. I have tried to make an in app purchase but I couldn't. It says my payment method was declined. I have funds in my bank so my partner tried her bank details instead and it has let us get as far as confirming purchasing but then it says 'Your Purchase Could Not Be Completed for assistance contact iTunes support at www.apple.com/support/iTunes/ww/. ' I don't understand. As far as we know we have set everything up as it should. We've connected the phone to iTunes. I don't understand. I've sent a message to support but seeing if anyone else has had this issue?