Is kJams currently compatible with Catalina?
No. You can not run kJams under macOS Catalina.
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 update: Fusion does NOT support extended desktop. Then install macOS Mojave (you can get it from the App store, even under Catalina). There is a forum discussion here. You can make a bootable Mojave Installer USB stick using DiskMaker.
Can I downgrade back to macOS 10.14 Mojave?
If you did NOT just get a brand new 2019 MacBook Pro, then YES, in two different ways: