If you see a USB device detection event in your system log but there is no ttyUSB[0-9] device it may be the CH340 USB serial adapter problem. There is a braille terminal based on that adapter and you may have a configuration for that even if you don’t need it. Unfortunately that configuration inhibits the creation of the ttyUSB device and Lightburn cannot access your engraver. Note the vendor/device ID and search in /usr/lib/udev/rules.d/* If an entry shows up edit that rules file, comment the line found and reload with udevadm control --reload-rules.