@grenade
In the flow where you send this automated message with the discount code you should add another email that will be the reminder for the code usage.
And about how to make sure only people who haven’t used it get the reminder this is a bit tricky and depends if this is a static code or dynamic one.
If it is static you can set up conditional split before email and set the condition to: “Placed order zero times overall where discount codes contain and pick that static discount code from the dropdown”
But if this is dynamic code that you are offering then the best you can do is set a simpler conditional split: “Placed order zero times since starting this flow” this way people who place order won’t receive the reminder even if they haven’t used the dynamic code but that is the closest you can do the setup to what you want to do.. Anyway if they placed an order you don’t need to send them the discount reminder..