why do i get a message 'your purchase could not be completed' contact itunes support for assistance ,contact ITunes support at www.apple.com/support/iTunes/website/.?

I Keep getting this Pop up and not allowed to pay for any app.i use Itunes card to redeem my account and do not share with no one its my personal account please help me solve this so I can be able to purchase with my account .as soon as this can work out because I was in the middle of my game when I got that crab(your purchase can't be completed contact iTunes support at .