Java applets aren't supported anymore in Chrome, and I think other browsers are following suit (NPAPI based plugins in general are being phased out.)
It looks like Itch does support it, though. Just keep in mind that the future of in-browser support for Java applets is sketchy, unless Oracle makes a native interpreter/whatever you call it for Java.
(Actually, if you are using Chrome that may explain the game not running, on my Chromebook at least it just shows nothing for the Java games on Itch.)