To create promo code:
1.Click Create Promo Code
2.Choose discount type
3.Set capping to the promo code
4.Set Start Date and End Date of when the promo codes will start and ends
5.Click on Ticket Type and Assign Ticket that you would like to apply the promo code.
You may also want to read the article related to Promo Code: Why am I seeing invalid promo code message?