Yes you can, but I prefer not to. The fridge will cool down the panna cotta more evenly, so that you get a consistent texture as it sets. If you keep it in the freezer, the outside could cool down and freeze faster than the center. This will give the panna cotta two different textures. Not appetizing. It's also possible that you could forget the panna cotta in the freezer, and end up with a weird-ass frozen panna cotta the next day.