I’m having the same exact issue after updating to WooCommerce 3.
When I try apply the coupon manually, it does give me an error.
It says that The minimum quantity of matching products for this coupon is 7.
But I have 7 of that product in my cart, so something is broken. I even tried changing the quantity in my cart to 8 and it still gave me the same error.
It worked perfectly before updating to WooCommerce 3.
Has anyone found a solution for this yet?