if using the bundled “popup” flash - make sure it is attached properly and flipped up 10-20 degrees
if using an external flash, ensure it is properly attached in the hotshoe and turned on, and ready light is ON
ensure RC mode is OFF unless you are using remote compatible flashes
ensure flash mode is not set to Flash Off
if using the E-M1 Mark II in silent shutter mode, check that flash in this mode is not disabled in the menu: Camera2: Anti-Shock/Silent:Silent Mode Settings:Flash Mode (note flash sync in silent shutter is limited to 1/50th sec)