Why always ask me on my post code

Have you entered your post code/zip code in your billing information?

Log into your account in the App Store/iTunes and you should be able to change/verify it there. Or you can click the blue Edit payment methods in the article below. When signed in and the page loads, you will be on the credit card page. After updating that, click the Go Back button at the lower left and you will be on the address page. .

Apple ID payment information - change.  *