All promotional Yubico coupons are only able to be redeemed once.
If you are receiving an error at checkout trying to use a coupon, please check the terms and conditions to make sure;
1) you are attempting to use it before the expiration date
2) the coupon is valid for the products in your cart
If you've confirmed that the coupon meets the terms and conditions and are still receiving an error, please take a screenshot of the error you see, as well as the coupon code, and submit a support ticket with this information.