Hi romeel_janjua,
Thanks for reaching out here in Apple Support Communities! We understand that you're trying to update your payment information, and are unable to add a credit card for some reason. To protect your privacy, we're unable to access any account details or determine why the card is being declined, however the article below may help.
If your payment method is declined in the App Store or iTunes Store
Some important items to keep in mind:
1.) The Apple ID region and the billing region for the credit card must be the same
2.) The billing address you enter for the credit card must match what the credit card company has. If you've recently moved and not updated your billing address yet, this could cause the card to be declined.
3.) The card may be declined if the credit card issuer hasn't approved it for online/electronic purchases.
If you're still running into problems adding the card, we encourage you to contact our Apple Support advisors at Get Support for more information.
Have a great day.