In the most recent firmware, Snapmaker presents itself as GRBL instead of Marlin. This is a good choice, as Marlin is mainly used in the 3D printer world. Not a good choice was to make the switch without giving the devs of LightBurn a heads up.
Please see: Announcement: Snapmaker firmware updates have affected LightBurn usage
You will need to add a new GRBL device by importing the corresponding .LBDEV file from here: Using Ray/Artisan/2.0 with LightBurn | Snapmaker Wiki