My name is Lilith, I'm on the Social Care team at Shopify.
If you've used the Discounts guide and you're encountering this message at checkout, there could be a few different factors that can cause this error. It could be a setting in the discount, time and date, customer actions and details at checkout, or other reasons.
I'd like to take a closer look at this if you'd like. I will have to send over an email to you to get you authenticated. Would you like me to check into this for you?
Let me know how you do with the information above! I'm happy to answer any questions you think of as you work on your business.