check the igniter or glow plug to see if its working, should be able to see them from the outside through the flame inspection hole, also if it has been sitting for awhile the burner tube may have insect or rodent nest inside it, If the flame sensor doesn't see a temp rise in a few seconds from turning on it will set a fault and go into lockout ,could also be a flames sensor bad.
I think it is probably the igniter not lighting or a nest blocking the lp from lighting up.
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