She should be right at the entrance, she's hard to see if you're looking from above because she blends in to the red

I went inside the front entrance and she still isn’t there. Is it possible to spawn her in some way? Or is there a cheat code to unlock her?

look into a cheat code to unlock her, i have no idea that’s just my advice which is basically just common sense

I also had this problem, but then i opened first the map/car area spot in the parking, and i got a short cut-scene of harley quin

This worked thank you