Hello,
Sorry but I just process a test and I see the error message... And it seems that you are on Url link you provide is on HikaShop 3.2.
I believed that the issue is that you don't have coupon error message on HikaShop 3.2.
All of this isn't clear can you precise the idea ? From our side we have never be process on a website that don't display error message... From this help you is impossible.
Now, maybe for a reason or another, you test domain haven't the issue ? And your product site have it ?...
Can't say much more from now.
Regards