Gas hot water heater will not automatically heat water, i have to manually start the burner to heat the water

it is a WATER HEATER. you do not need to heat HOT water.
Gas or electric?
Electric = turn power on
Gas = instructions are on gas valve or in your manual.
does not light = verify a spark at burner when you push button (can be hard to see, turn off lights), verify gas valves are on and another appliance has gas, call for service.

Troubleshoot service manual by type of gas control valve
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
Post question on following link:
http://www.thetankatwaterheaterrescue.com/forums/forum3/
Add a comment and say what you discover

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) Rheem does not post service manuals.
Call Rheem and ask why, since the other big manufacturers, AO Smith and Bradford White do post service manuals.

2) You need new propane gas control valve, and matching combustion parts to fit existing tank.
http://waterheatertimer.org/pdf/Rheem-Water-Heater-parts-catalog.pdf
http://waterheatertimer.org/images/IMG_1826-LP-burner-450.jpg

3) Rheem does not recommend changing gas type, especially from natural gas to LPG.
LPG is higher pressure, and burns much hotter and requires cast iron burner, unlike the lighter-weight metal used for Natural gas burner.
The manifold tube and thermocouple must match the gas control valve, and gas-type and have correct thread for propane.
The combustion chamber and heat transfer surface can also be heavier metal for propane heaters.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Hello Mike,
Check gas pressure, burner parts need to be cleaned, check electrical outlet, hot surface igniter is bad

White Rodgers intellivent error code:
http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html#Intellivent
http://waterheatertimer.org/pdf/Intelli-vent-gas-valve2.pdf

Error code says: system in lockout, but you know that.
1) Low gas pressure, no gas to water heater: call gas company to check pressure
2) Hot surface igniter not positioned correctly: inspect and clean burner parts.
http://waterheatertimer.org/images/Reliance-hot-surface-ignite.jpg
3) Low voltage to water heater/ outlet has wrong polarity: Use circuit analyzer on electrical outlet to make sure polarity is correct

Typical power vent manual:
http://waterheatertimer.org/pdf/Reliance-40-50-power-vent.pdf
Cleaning water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#clean
http://waterheatertimer.org/pdf/How-to-clean-combustion-chamber.pdf

Is this gas or electric heater?

Gas heater, the thermostat might be bad.
You might need to change out gas valve.
Also look in and see if flame is light blue or yellow.
Blue is good.
Yellow flame means you need to clean burner, or replace burner.
Burner should be cleaned each year.
Refer to manual for instructions and parts list.
http://www.whirlpoolwaterheaters.com/support/Manuals.aspx
Also tank might be filled with sediment.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Electric water heater is different altogether:
Some hot water means upper element is working.
Water heater ECO reset button is good.
Tank is not overheating.
Electricity and circuit breaker are good.
Remaining suspects:
Thermostats are bad, loose wire, shorted wire, lower element burned out, sediment inside tank.

I test instead of guess:
Here is 30 minute troubleshoot overview that will identify problem and show how to repair electric water heater.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

The first thing I would do is turn up temp control on gas valve.Control is located on front of gas valve at the bottom front of heater.They usually have a indicator dial on front with an arrow pointing direction.Turn dial to higher setting when burner is not on and this should turn burner on and provide hotter water. The control is normally turned clockwise to increase temperature.If control is already turned all the way up then water heater may need to be replaced.Depending on water quality in your area heater may have a large amount of mineral build up and it may not be capable of producing enough or hot enough water. If this the case all I can suggest is to try to drain tank and flush some of the deposits out. If you have installation manual it will give you instructions on how to do this. If not see if you have working shut off valve on cold water line ,if it works shut it off and connect drain hose to drain valve on bottom right side of gas valve. Put hose to floor drain or sump basin if you have one. Turn control knob on top of gas valve from on to pilot,this will leave pilot light on but prevent burner from working while draining. Open drain valve ,flow rate may be slow or not at all. Open hot water side of some faucet to provide air to help draining. You may try opening water supply valve to heater a few times to aid in draining. If you can get it to drain,shut drain valve off when done,turn cold water supply back on and shut off hot water outlet. Then when tank is full open hot water vavle to bleed air out of heater and piping. Turn control knob on top of gas valve back to on position. This should bring burner back on .It will take about 1/2 hour to heat. If you can't get it to drain or this doesn't help you might consider getting a new 40 gallon heater. Hope this helps. Thanks

Check thermocouple , sounds like it's not sensing flame.

gas regulator gone bad

LP OR NAT GAS , IF IT IS LP CHECK GAS PRESSURE @ TANK AND MAKE SURE NOT TO HIGH , IF NAT STILL COULD BE THE PROBLEM HAVE THE GAS COMPANY CK PRESSURE