Why is my floating mob farm not working?

Check if you left any source of light inside and check if you are in the middle of a body of water, mobs tend to spawn more when they are away from an island so you may want to consider taking it down and building in in the middle of the ocean. You should also AFK 20 blocks away from the farm. Make sure that your farm is 128 blocks high from any other block and 128 blocks far from any potential spawning points. Mobs also only spawn if you are in close proximity, about 240 times 240.

If you do play in PI version then notice there are no mobs.

I assume you must have noticed the very limited gamearea as well as other limitations.

However it is cool it runs on that piece of hardware.

Well, first of all, make sure to light up all the caves in the surrounding area, as it pushes mobs into your farm because there is nowhere else to spawn them. Also, check if you are on peaceful.