https://link.springer.com/referenceworkentry/10.1007%2F978-3-319-08234-9_272-1
It's the architecture, the server does all the logical shenanigans, and the client is only a viewport, an interface to the state that's managed in the server side
Some zip managers like to take control of the .jar file extension away from Java.
Make sure you have Java installed and try running Jarfix once, then try the installer again.
I seem to recall getting this error myself... I think it had something to do with caps, also I do what GitHub suggests and put those in a gradle.properties in my user folder so they're global and there's no risk of accidentally committing them