The very tiny server is getting a bit crushed at the moment. If you actually want to try it out, check the tutorial tomorrow when the traffic goes back to normal.
I encountered the same error message, yet it seemed to be from a broken link instead. From the homepage [0] while signed in, I clicked on "How to play" right under the query entry box. It sent me to this page, which is a different url from the one you linked: https://wiki.github.com/Abstrct/Schemaverse/how-to-play
0: https://schemaverse.com/tw/index.php