my phone keeps asking me to update payment information bit its already correct and because of this it wong let me download the WhatsApp

A “payment declined” or “card 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:

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

Here is additional information from Apple→If your payment method is declined in the App Store or iTunes Store - Apple Support

Sorry about that Laurence - my response was not meant for you; I posted it in the wrong place. My apologies. BTW, I have since found a solution to this problem; perhaps it can help you as well. I logged in to Apple iTunes, accessed my Apple account from there. It would not allow me to delete the card in question because it was the only one in my account, but I added another card, then deleted the original card. After that, I re-entered the original card and left the new card there with it. The fault seems to have disappeared, at least for now.

Roslou wrote:

my payment details are all uptpdate?

Apparently, that is incorrect.

If your payment method is being rejected, contact iTunes support.

I have a recurring "Update payment information". My iPhone was working just fine until I updated to version 14.8.1. Ever since, this error has popped up regardless of how many times I tried to re-enter my credit card information.

Have it your way. But it won’t solve your problem. I’ve told you facts. You aren’t required to believe them; you are free to have your own fantasies. Have you bothered to check all of the possible causes I listed, or are you just responding to my first sentence?

Have you checked with your issuers? Have you contacted Apple?

I’m glad you solved it. That was a bit strange!