That failure tree has several branches. Could be a bad neutral start switch, missing switch, wiring failure, internal failure inside the transmission, etc.

A visual check is the first thing. See if the wiring is hooked up, is the switch installed, etc. You can check the switch for operation with a voltmeter. If the switch isn't working it could be a bad switch or it could be a busted rooster comb. Might need to pull the pan off the transmission if the string of clues leads you there.