Check your display number
ps -ef|grep Xvfb
00:00:00 Xvfb :99 -ac -screen 0 1280x1024x24
as here it is 99
in your Jenkins script use this as display number
like
export DISPLAY=:99
reference doc: xvfb(1): virtual framebuffer X server for X - Linux man page
you can control how the xfvb is started.
how to actually do it, depends if you are using jenkins xfvb plugin, or do you start/pass the options via third-party scripts.
however make note that the $DISPLAY value should match in both, the daemon launching script and in your katalon execution script.