Check the following:
There may be insufficient funds on your card. You can check your card's balance in your Nimble Member Area or call the Nimble team on 133 156
Your card may have expired. Check the expiry date on the front of the card.
The magnetic stripe or chip on your card may be damaged. If that's the case, call the Nimble team on 133 156 (during business hours) to get a replacement card.