Once you have created your coupon you then need to attach it to the Membership Types and/or Program Session you want purchaser to be able to apply it to.
To do this, select the row that contains that coupon, it will highlight in blue. Once selected you will be able to checkmark which Membership Type or Session this coupon can be used on. The coupon can be attached on multiple Types and Session and you can have multiple coupons attached to the same Types and Sessions.