T-stat contacts make up........fan comes on then spark electrode fires to ignite main gas......
BUT yours is NOT lighting main gas.
Various causes from voltage to bad board
Light off ALL stove top burners on high to get propane flowing and place a demand on regulator.
If flames are steady, strong then pressure is OK
If flames fluctuate and diminish with them all on then regulator is not maintaining proper pressure and needs adjusting.
IF flames are steady...try furnace a couple times to make sure problem is not just lack of propane flow
If it still doesn't light off.......start with a good cleaning of burner assembly and check of spark electrode.
here is a service manual that includes your model.........pg. 30 outlines causes/solutions
LINK