Customers receive the message above when entering a coupon code even though they have spent more than the miniumum amount.
Replace the file /includes/modules/order_total/ot_coupon.php with the one located here ot_coupon Zen cart v15
Source and for further info visit: http://www.zen-cart.com/showthread.php?195561-Discount-Coupon-Restrictions-question/page2
Always backup your site before making changes