You have not completed the entire form

Someone else has posed previously that: I’m trying to update my payment details, each time after adding payment information I get an error message saying “you have not completed the entire form”. I have the same issue and the response that "you need to fill in all the details" is not helpful. So here is one which is:

The field you are likely missing is suburb, which is marked as "Optional" but isn't. The form, on Ipad, does not highlight the mandatory field when it is submitted but fails validation, nor does it indicate in the error "You have not completed the entire form" which fields need completion.

This is an issue which should be corrected by Apple and should have been picked up in QA. I was only able to resolve it by trying to add the payment method in ITunes and noting which field was highlighted on submission.