Hi @Ryo,
Welcome to the Community! Thanks for posting your question here regarding discount codes and emails.
Unfortunately, Shopify does not pass back the information to us about whether or not a coupon code has been used. Because of this, we are not able to use discount code in a reminder flow like this. To create a flow reminder email, I would recommend using a 'Has Placed Order zero time since starting this flow' flow filter. This will work to remove users from the flow as soon as they have placed an order (which we can assume they would have used the discount).
Have you considered creating a flow for those who have received a discount code and making a reminder email send after using the “ wait X days” step in the flow? More guidance can be found in this article here!
Thank you and have a good day,
Alex