To 'reset' you have to turn ON/OFF switch OFF then back ON
If heater has been unused you should:
Open hot water faucet to purge any air from lines. Then close it when you get water flow and insure you have water pressure
Turn 12V DC power on.....Heater Switch ON
LP gas valved in
Open hot water faucet to establish flow
(may have to open/close hot water several ties to establish LP flow to heater)
If it fails to light and locks out then you have to turn switch off/on to reset
If no operation check the 3A in-line fuse
HERE is manual