You want X sync for electronic flash.
M is for M series flashbulbs of yore…

Try this for fun.
Take the back off and set your lens to 4.5 or whatever the widest is.
Fire your shutter with strobes connected at a white wall and observe through the lens from the rear with the sync switch on different settings.

The M has a delay so it allowed the bulb to burn up to full intensity.
A strobe is instantaneous so the shutter opens too late with an M setting.