Check the store settings, Google, Apple or Amazon, whichever you use.
In app purchases can be disabled as a parental tool.
Is your debit/credit card vaild?
Are you using giftcards and live in the US? If so have you added the tax to the total sum?
You can not use a $10 giftcard to buy the $9.99 pack as it will not cover the additional tax.
Either choose a smaller pack or add another giftcard so the taxes are covered.
This Forum is provided by EA but is designed as Player helping/advising other Players.
Contact EA support
http://help.ea.com/en/contact-us/new/ and ask for a gameadvisor. Same login as the game.
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+++ This Forum is provided by EA but it is designed as Player helping/advising other Players - meaning we are not employed by EA +++
D'oh! - Em