Older versions of the Gravity Forms Coupons Add-On require a working product field to be present on the form – ensure it’s editable.
If you have Gravity Flow 1.4.2+ and Gravity Forms Coupons Add-On 2.3.2+, the editable product field is no longer a requirement. The coupons field will now function even when it’s the only editable field on the step, as long as the display fields setting is set to all fields, or there is at least one product or total field selected.