If you have imported an image file such as pdf you will not get an option to use the line since there arent really any lines, so you get the default of ‘fill’. You may need to select your image, then use the ‘Trace Image’ option in Lightburn which will then trace the image for you. Make sure you then place the image on a separate layer, and place the line on it’s own layer also, then you can work with the line to do a cut on the line. From there you can have either layer set to Output or not.
It may be difficult to see the line at first. The trace option is instantaneous so you dont see it happening. You may need to ‘move’ the image to see the line, then put it on it’s own layer.