I have tried adding different cards for my payment method but it keeps declining why

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, you need to add first the new card, then remove the old card.

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

The message "Your your payment method is declined in the App Store and iTunes" disappears if the above succeeds.

If you see a message that says "Your your payment method is declined in the App Store and iTunes", there is a problem with your payment method on your account.

See -> If your payment method is declined in the App Store or iTunes Store, see -> 

If your payment method is declined in the App Store or iTunes Store - Apple Support

Scroll to:

• How to complete your purchase if a payment failed or was declined
• If you still can't make a purchase
• Still need help? Contact Apple Support.