I wanted to game with the mod and tried to install citadel, but it wouldnt start with a double click. So I googled a bit (i dont know anything about coding) and found out, that i could try start it manually via a command screen.
Got that screen open, typed to command and got "No Main-Class found" as an result. Googled that one too, found some ideas how to fix this.

But I cant get it fixed. There is no class data in the jar-archive of Citadel and so I dont know what to add in the manifest as the main class. Can someone help me to find the missing piece? seems like i am the only one with this problem.

The Main Class was missing right after the download, I didnt click or do anything other than "download".

Would be gratefull for help!

Mate, you don't install mods as executables by double clicking on them. The .jar files aren't meant to be run that way. And you certainly don't try to run them via command line. You need to install either Forge or Fabric which are mod loaders. Citadel only works with Forge, so you'll need that. I'd look up a basic tutorial on how to install Forge and Forge mods if I were you.

i didnt know that, it sounded like citadel is an application like optifine and stuff. But thanks, will do that. Like I said, I dont know shit about coding and stuff. So I just needed someone to explain it to me.

Ah now I see why you're confused. Optifine's different. They went out of their way to make their jar file work as an installer that you double click on. Most mods aren't that fancy lol.