Hi Elton,
Sorry for the late reply. I believe the problem is that for a coupon to be applied for a guest user, there must be something added to the cart before that, i.e. coupons can not be applied to an empty cart for guests. I will have to find a solution to this – will get back to you asap. Meanwhile, it looks like you can “solve” this by adding some product via URL at the same time when applying the coupon. For example:
http://example.com/cart/?add-to-cart=2495&coupon=test
Also, in Pro version, there is “Add products to cart” (for “Fixed product discount” type coupons) option, which seems to be solving the problem for “fixed product discount” coupons at least.