Monterey (and later), and mac ARM
Is kJams currently compatible with Catalina?
No. You can not run kJams under macOS Catalina.
Why not?
kJams is currently a 32bit application, and Catalina does not support 32bit apps.
When will kJams be able to run under Catalina?
The hope is "some time in 2020", the optimistic hope is "mid-2020", but beware "the planning fallacy"
How can I run kJams today if I have Catalina?
You can use a Virtual Machine such as Parallels Desktop, or VMWare Fusion, and install macOS Mojave (you can get it from the App store, even under Catalina)