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This usually happens if the billing address of iTunes different from credit card address.
You need to re-enter the card from your iTunes account and add it again. You can change the card details from iTunes on your computer.
Download and install latest iTunes on your computer from here --> http://www.apple.com/itunes/download/
Then follow the instructions in here --> https://support.apple.com/en-in/HT203208
If you have any difficulty on following the steps above, please let me know and I will continue to help you.
You can also update the card details using your iPhone itself.
Let me know how it goes.