The gas valve should have 24vac and be open for about 6 seconds, before it shuts down IF flame not sensed at burner.
Check burner tube and burner orifice/spud behind gas valve for blockages (spider-web-bug).
IF you have a remote flame sensor, clean it carefully with steel wool.
The pressure switch closes once it reaches set water column pressure.
Be sure combustion air pipe is not clogged or separated/leaking, combustion motor and wheel are clean and running strong and tubes are not clogged, kinked or loose.
Be sure ignitor is getting full 120vac for ignition cycle.
You did receive the wrong switch, will need to return it.
Correct switch is 024-27666-001 (new p/n).
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?