Hayward propane pool heater service light on Lo 60 message

"LO" is simply informing you that the Heater is not sensing proper water flow through it! A Flow Problem.
A special sensor, called a pressure switch is this source. But... That does not mean it is bad - No, it is in most cases just doing its job!
Most common reason - Dirty Filter.
A good cleaning of the cartridges, or maybe a breakdown and recharge of a DE Filter, or a good cleansing backwash of your sand filter, will remedy this situation.
Or... Check for any OTHER reason or situations water flow may be reduced through the heater:

• enough water in pool or spa?
• If you have a weir gate in skimmer, is it stuck in the UP position?
• Are any valves closed?
• Do you have a bypass valve at the heater?
  • If open, this will reduce the amount of water going thru heater, close it to the point, LO goes away.
• Etc