How do I improve the Magento coupon code error messages?
There's an extension which improves upon Magento's standard coupon code error messages. It can be found here:
It will tell you exactly why the coupon didn't work whether that's a customer group condition, subtotal condition, etc. And I integrated with it to also show MageMail auto expiration dates.
Note that we cannot vouch for the extension's ongoing performance, maintenance, and backwards compatibility. Please do your due diligence before using the extension.