Coupon codes must be unique across all coupons (active or inactive). When you save, Sprwt looks for another coupon with the same Code; if it finds one that is not the coupon you are editing, it blocks the save with "That Coupon code already exists!". Either pick a different code or edit/reactivate the existing coupon instead. Because coupons cannot be deleted, an old code stays reserved unless you change it on the old coupon.