Pilot light wont lit up

Change the gas valve.

Tripped FV or TRD, or bad gas control valve, low gas pressure, air in gas line.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
Unfortunately, Rheem does not post service manuals. Other companies post service manual, but only Rheem uses TRD FV system, while other companies use arrestor screen.
http://waterheatertimer.org/How-to-repair-Rheem-TRD.html
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) Copy following link:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

2) For tank-type gas:
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#pilot
http://waterheatertimer.org/How-to-repair-Rheem-TRD.html

3) Add comment and say what you find so far.

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

If I understand correctly, the pilot light is not staying lit.
You light the pilot and water in the tank reaches temperature.
Once hot water is gone, then pilot light has to be lit again.

Gas pressure is good, and gas is flowing.
Thermostat is working at high end because it cuts off heating.
Vent is working because tank heats up.
Air intake is working.

Gas control thermostat might not be reading low temperature. Replace gas control valve.
Pilot tube might be dirty. Clean burner parts and combustion chamber, and air intake, and vent.
Backdraft might be blowing out pilot. Check for constant updraft by running water heater 5 minutes, and then light match under vent hood and see if smoke goes up.
Very hot air can put out pilot, especially when water heater located in attic. Add ventilation.
Improper vent might have water dripping back down into combustion chamber and putting out pilot light and rusting out the parts. Inspect for rusted parts and water.
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf

Following information is for tank-type gas water heater:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/pdf/Pilot-light-will-not-stay-lit.pdf
http://waterheatertimer.org/pdf/Attic-Ventilation-and-Pilot-Outage-in-Gas%20Water-Heaters.pdf
http://waterheatertimer.org/images/Water-heater-in-attic-800.jpg

Hopefully just the thermocouple. The thermocouple is the small rod right next to the pilot light. The flame on the pilot heats up the thermocouple and keeps the gas valve on. You can replace it yourself. The thermocouple will be connect by a small nut on the underneath side of the valve, and you will have to pull the thermocouple out of the pilot assembly. Not an expensive part, you can get it at any hardware store. Hope this helps you .

there could be a small blockage in the pilot light gas nozzle which just need cleaning

Hello Make sure the gas is on and be paitent you have to purge the air out of the pipes/tube to light pilot.

The most common reason a pilot light will not stay lit is a bad thermocouple. How old is the water heater? If the pilot light will light, but then go out when you release the safety button, that is a sure sign the thermocouple is bad. You can buy a replacement thermocouple at your local home improvement store. There is a small chance that the control valve has gone bad, but that is highly unlikely.

1) Open the door on the bottom of the tank to show the burner. 2) Turn the valve to "pilot" and depress. 3) Light the match, (best if you have a candle/grill lighter), and follow the skinny copper tube from the valve to the burner inside (this is called the thermocoupler). At the end of it, you will see the pilot. 4) Once pilot is lit, keep valve depressed for a minute (this warms the thermocoupler, thus sending a signal to the valve saying the pilot is lit). 5) Let go of the pilot button (usually red). 6) If pilot stays on, put the door(s) back on and turn the valve to "on". You will hear/see the gas ignite when complete. Good luck!