Gottcha.
Same type of unit in my shop rescued from my parents home remodel. It sits idle all spring/summer, puts on the pilot, but no main flame.
Did this for a couple of years. Finally determined that I had to leave the gas on all year long to the regulator. There are some diaphragms in there that get hard or something, and don't open.
I know this is redneck, but whack it a few times and see if you can unseat the diaphragms. If not it might take a kit in the regulator.
Yea, I have an old john deere heater that does that, one day that thing is gonna quit..... 8)