I got something similar, to the original game. I found a tutorial on Reddit for an experimental version. Works normally like the original Clangen. It's a bit long of a long tutorial yet not hard. Here: GUIDE ON HOW TO INSTALL AND PLAY CLAN-GEN EXPERIMENTAL VERSION : r/WarriorCats (reddit.com) It might work. Maybe it can download better? If not I'm outta ideas, hope it might've helped!
"Experimental version" has not been a thing for several months now and is now called developmental version. You also cannot get official tech support on the discord with source versions, and this guide is generally a bit outdated and not really the best way to play the game, even if you want to play source versions.