What could it be ?
Can you share your GCode? You can save it by clicking Save GCode in the laser window or going to ‘File’ > Save GCode, then upload it here.
Have you added anything to your Start GCode block in your Device Settings?
Ensure you have the correct grbl in the device setup… Lightburn is sending a code that it thinks is invalid. I’m assuming you have the device ‘type’ incorrectly configured.