Everytime I try to download an app from the App Store it keeps declining the payment

A “payment declined” message means that the issuer of the credit or debit card you have on file in your iTunes account rejected a charge or account confirmation. It is not an Apple problem; Apple is just passing through the message from your issuer. There are a number of reasons the charge or verification might be declined:

1. Insufficient balance or account in arrears
2. Account suspended for suspected fraud (not necessarily by you)
3. Error in card number
4. Wrong expiration date
5. Wrong security (CVV) code
6. Address in your iTunes account does not match exactly the billing address for the card
7. The card was not issued in the country of the iTunes store
8. You are connected through a VPN provider that does not terminate in the country of the iTunes store you are connected to.

You need to have a valid payment method in your account, even for free downloads. If you have no apps with in-app purchases and no subscriptions you may not need any payment method, but if you do have one it must be valid.

Not all debit cards are accepted as a payment method. See this, Payment methods that you can use with your Apple ID - Apple Support

It needs to be issued in same country as your account is registered to.

[Edited by Moderator]