Your flow switch is a magnetic "sail" switch that is physically closed by the moving water. No moving water = a flashing FLO. The flo switch is located at the upper right EQ panel and is transparent so you can see the magnetic finger that closes the connection when the water passes through. Find out why there is NO water passing through and you have found your problem.
scenario #1, clogged or dirty filter restricting adequate water, solution remove the filter and see if it changes.
scenario #2, check the "grid" screen at the filter suction fitting inside the filter bay for collected debris, i.e. leaves etc. blocking suction, solution is to clean grid screen.
scenario #3 still no water flow, put your hand on the circ pump and see if it is 1) working, 2) hot to touch, 3) no water flow, solution = a)air lock, b) suction blockage, c) impellar debris inside
The "flo" switch is preventing the heater operation because of "lack of, or low water flow"