i am unable to update my billing info it just keeps giving an error

See these Apple documents.

 Change, add, or remove Apple ID payment methods https://support.apple.com/en-us/HT201266

If you need additional help, see Subscriptions and Services https://support.apple.com/en-euro/billing

If you still are having a problem, use Helpful topics https://support.apple.com/contact

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.

iTunes Store - Contact Us - Official Apple Support. https://support.apple.com/en-euro/itunes-contact

iTunes Support – world wide https://support.apple.com/choose-country-region/itunes

1) Check if the card you are using it is internationally enabled if you are living outside US. If it's not, check how to enable it for you card through your respective bank. If it's already enabled, then the issue might be different.

2) If the card you are trying to add was used on another Apple ID previously and still a payment method on it, and that the Apple ID (the Apple ID on which the card was used before) has some debt that need to be paid, then the card can't be used on a present Apple ID (the one you are trying the card to) until the debt on the old Apple ID is paid. <This was my issue> So, try to complete the payment on the old Apple ID using the same or other card and try to add the card to new ID.

If the above haven't solved the problem, contact Apple Support.