Parking violation

I go to UHD. About 2 weeks ago on November 11th I parked under the bridge on the main campus like I always do. I went to the Park Houston app and tried paying for my parking, but the app wasn’t letting saying that there is no charge for the 2 hours so I took a screenshot. Today I was only in class for an hour so I didn’t try to pay thinking that it wasn’t going to let me. I come back to my car and there’s a green envelope with nothing in it. So I didn’t even realize what it was until I actually read it. Should I try to appeal the ticket or just pay the $30?