Can you use a Harkins gift card to purchase online tickets?

I'm trying to buy tickets for the upcoming Harry Potter film. I'm on
movietickets.com I checked how many tickets I want and I checked the
box that says "Yes I would like to add a promotional code or
Movietickets gift card" I put my harkins gift card code in and it says
its invalid. Does this mean I must use my credit card to buy the
tickets online?