When a Coupon Code is used, the code can be found in the "Details" column in the Customer's Transaction History, like this:

Coupon Codes are also saved under the default Discount "Misc Credit," which cannot be found on the Discount Report. Therefore, at this time, the best way to create a Coupon Code report is to run a Transactions Report and export it. Follow these steps:

Run a Transactions report for the desired date range. Click here for instructions on how to run this report.

Download the results to a CSV. Within Excel, filter to show only the rows of the type "Misc Credit." The Coupon Code will show up under the column "Invoice Memo."