The NEJE laser requires a solid ground and a +ve PWM signal. If you insist on using the SKR fan output you will need to invert the signal. Some people manage to get their lasers working using the FAN outputs by feeding the laser with the fan output to it’s power lines thus modulating the entire laser control board and laser diode. Not something I would recommend. A fan doesn’t care which leg gets modulated but the electronics in your laser control board will. Probably the easiest solution would be an opto-isolator.
