First a caution. BEFORE you turn that switch from OFF to ON on the water heater itself, turn a hot water faucet on to fill the water heater.
Don't know if I can help, but have you checked to make certain your propane is on? If it's on, bleed the lines by lighting the stove to purge the lines. Then try hitting that water heater switch to ON.
2015 Dynamax REV 24TB class C