When coupon code entered and "Apply Code" is clicked, the coupon is not applied

I have a client with an ecommerce site using MarketPress. She just set up a coupon and sent out a newsletter with the coupon code, but the coupon isn't working. I have copied the live site to a staging site and the problem occurs there as well. I'm granting Support Access to the staging site: http://spiritdancer.staging.wpengine.com

This is urgent. She has gotten emails from customers complaining about the coupon not working.

Thanks,
Richard