Make sure you have 2-Step Verification enabled. Your account is REQUIRED to have 2-Step Verification in order to purchase Protobucks and/or NCoin.
If you’re receiving a message indicating that you need to “Add Additional Funds” when attempting to purchase Protobucks, this means you’ve never purchased NCoin or Protobucks before via the In-Game Store. You will need to complete at least one purchase of NCoin via your NC Account in order to purchase via the In-Game Store moving forward. To do so, log into your NC Account and click on the BUY NCOIN button on the top of the screen.