Good day, Eve2019.
To make verify if a discount is taxable or not, you'll want to reach out to the sales tax department of your State. Then, modify the item's tax code by following these steps:
- Go to List > Item List.
- Double-click on the item to edit.
- Select the Tax Code (Taxable or Non-Taxable) of the discount.
- Click Ok.
I've got this article for reference: Add, edit, and delete items.
Please let me know if you have any additional questions or other concerns. Take care and stay safe.