trying to update my payment information but it says "payment info could not be updated. payment method was declined. enter valid payment method info

In this case, your only option is to contact iTunes Store support directly.

You can start with the Contact Support button at top of this page. In my experience, the easiest (and fastest) way to contact Apple these days is via chat on their downloaded Apple Support app.

When it says the card was declined, it’s almost always your bank that declined it for use on iTunes/App Stores. You will have to contact the bank to find out why.

To read more about declined cards:

https://support.apple.com/en-us/HT203005

Did you contact your bank?

There have been many scams lately that fraudulently bill credit and debit cards pretending to be from iTunes or App Store. As a result, many banks do not automatically authorize their cards, until they’re contacted directly.