I’m going to take the liberty of swapping the message body and title of your post.
This is usually caused by a computer not being connected to the laser engraver, or another piece of software hanging on to the USB port and not allowing LightBurn to communicate with it.
If you’re working with an xTool D1 Pro engraver, please confirm that XCS has shut down properly then reconnect with LightBurn.
I would be interested to know if you have performed an update to the xTool D1 Pro as another user found additional lines added to their work and attributed the addition to the firmware update.