Hey Rakassan! I understand that Subway has a rewards card and a gift card. To pay for credits you will need a Subway gift card and not rewards points.
If you are using a Subway gift card make sure that you have money on your subway gift card before continuing using the card to purchase credits.
If you continue having issues contact OpenBucks who is in charge of these types of purchases.
http://www.openbucks.com/contact.html
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