I assume the pen is controlled via a servo? (A link to the exact machine you have would be very helpful - there are SO many Eleks machines)
Basically, LightBurn doesn’t support pen plotters if they use the Z axis for lifting and dropping the pen. For Grbl we will only output M3 or M4 to turn on the laser and M5 to turn it off. But my guess is that your pen position is controlled by the Z axis instead of a simple on/off signal intended for the laser. That is how the vast majority of Grbl pen plotters work.
So unless you want to load custom grbl firmware on there that controls the servo via the laser on/off signal, LightBurn will not work for you. We do not support substituting Z axis movements in the place of laser on/off.
I recommend this version as a starting point: GitHub - cprezzi/grbl-servo: Special Grbl version for use with servo instead of spindle or laser.
The only other way I’ve found to do it is to intercept the laser PWM signal with another micro-controller and use that to control the servo.