My iPhone keeps saying I need to update my payment information

My phone keeps saying i need to update my payment information and every time I do try and update it it keeps saying that it has failed and that my payment method was declined even though it works with apple pay it does not want to update with the icloud and my card is still valid and there are no errors when writing the information down. Your systems keep failing to update the card. I tried linking the payment method with apple pay but it does not want to link and still fails.

[Re-Titled by Moderator]