I see you identify you have an Eleks on com3. The firmware and version would be helpful as well. Some of these machine ship with an older version of grbl so you may need to go with the GRBL-M3 device in LightBurn. Ideally, you would upgrade to GRBL 1.1f and then use just the GRBL device.
With GRBL 1.1f - Check your setting for $30 and $32 in the console by typing $$ and pressing enter.
$32 should be 1 (enable laser mode)
$30 should be 1000
The $30 value can be set either in your firmware to match LightBurn or in the LightBurn device settings (S Max) to match your GRBL setting.
Here is another great resource for your setup.
As another member previously posted, "Scroll down to the bottom "operating instructions" And follow that, once you have it working in elekscam. It should work in LightBurn. Follow the instructions to the letter and that will get you up and running…"