Payment method in Fiji.???

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.  

See Payment methods that you can use with your Apple ID https://support.apple.com/en-us/HT202631#africa. It shows that most credit and debit cards can be used in Fiji.