First, locate the flow switch in a transparent pvc "Tee" in the plumbing leaving the heater and crossing the upper section of the right service panel. You can see the operation of the sail switch as water moves through the switch and you should see it close. If it is not closing, you don't have water moving through the plumbing. When water moves through the Tee is closes the magnetic switch. YOUR switch is not closing and is the reason why you are getting the message.
Secondly, check the circ pump that drives the water through the heater system that will not work until the flow switch closes, signalling the flow of water that allows the heater to work.
Is the pump hot or cold to the touch. Check voltage going to the circ pump. It is likely to be a bad circ pump.