There are two graphical environment in ubuntu: xorg (old one) and wayland (new one). On my computer, Cafetran runs on xorg, but not on wayland. Cafetran is not supporting wayland?
Yes, you need to run CafeTran on Xorg for now as currently Java (the programming language CafeTran has been written in) does not run on Wayland (https://bugs.openjdk.java.net/browse/JDK-8210411). It may be fixed in the next Java 12.