Coupon allows free access, clients still have to put in their card details

I‘m repeatedly asked by clients who have a coupon that allows them to join a certain course for free, why they still have to put in their payment details. It seems like whenever you switch a product to paid, you cannot have a voucher that gives the product for free - clients still have to enter their payment details. The reason I’m not having 2 separate groups, one paid, one free is so that I I don’t always have to tag both of them. Fixing this would save a lot of time.