The Javascript exceptions in the console can give a hint. One problem seems to be that your scenario autoplays music, and the browser refuses this unless the user has already interacted with the page. But even if you do interact straight away, it seems that it fails due to the line: in Platform's constructor. I suspect the problem may relate to the leading slash, as it then tries to load the image from https://www.greenfoot.org/world/flat-tile.png which isn't going to work. I think if you remove the leading slash (to make it a relative rather than absolute path) it may resolve the issue, but I'm not 100% sure if images in subdirectories works correctly with Greenfoot's bundling and the online image resolution. Maybe someone else who has tried it in their scenario can share their experience.

I believe the problem is that you left all the System.out.println(...) statements in your code when you uploaded it. Try removing them first (or comment those lines out of the code).