Rheem gas heater unit won't fire up

Sublime Master ACtechguy imparts some pearls of wisdom to us on ignition problem with this furnace.  Let's listen...

If you have  a pilot flame that ignites and no main burner firing, then you have one of 3 very possible problems.
# 1 . if you have a separate spark module(separate from the main circuit board), then maybe the spark module is malfunctioning.

# 2  if the spark function is part  of the main circuit board , then your problem is the main circuit board.

# 3  Main burner side of gas valve is failed.

All Post 1990 spark ignition systems that I have seen (in my neck of Louisiana ) work in the same way.  They spark(sometimes for a long time on older models ) , Pilot flame ignites, then (sometimes  after a delay) , main burners ignite.
Spark ignition systems may not have any sort of flame sensor  that you can fix or clean( Flame sensing is built into the spark sensing circuit and not a seperate rod on the end of the burners),   .......

SO, if  you get spark, and then pilot flame and then no main burners, put a meter on the gas valve and look for 24 volts ac when the gas valve should be opening. If you get voltage at that point then the valve is bad . if you never get voltage there it is the spark module or main circuit board( Suspect whatever  part  that makes the spark).

Don't confuse the fact that just because the unit is sparking that everything is ok with the spark circuit.
From what you are describing , the spark control is sensing very little or no flame signal from the pilot flame.
And Yes, Once the control senses a stable pilot flame , the main gas valve will be opened(this could be very fast , or it could take 5  to 10 seconds, older models sometimes had long delays till main gas valve opening.

There are several things you can check on the flame sensing circuit .
Spark systems sense flame via chasis ground. ANY CORROSION  will mess up the flame sensing circuit.

Make sure all ground connections to the control are good . clean all connections to board .. Clean any ground wires to burners. Pull pilot assembly and clean all metal surfaces that the pilot flame contacts(there will be metal rods or bars that the flame inpinges upon) . The flame contacting these metal peices  is how the flame signal gets to ground and back to the control.

Blow out pilot tube with compressed air or something similar to clear out any blockages that may be reducing pilot flame size .

Still have the same problem after cleaning all this stuff?

REPLACE whatever device generates the spark.