How do I use the Commerce Coupon module?
The key to creating the coupon rules are two events: "Validate Coupon," and "Redeem a Coupon."

In this case, you need to create a rule with the "Redeem a coupon" event; in it, you need to add the actions "Create coupon line item," and "Set granted coupon amount" (in that order). The "Set granted coupon amount" action requires a number that is the discount to be applied. You need to calculate this in the rule by finding the percentage of the order total.

This video may help.