The sail switch and the control board requires a lot of work to remove the sail switch and the control board.
After the fan has started and run, the sial switch should close and then it will open the gas valve and the ignitor will start to spark and you can hear the clicking. If you are not hearing the clicking, you need to look at the sail switch, voltage at the board and the control board. If the ignitor is not properly located, it may or may not click and will not fire the burner.
If the board is bad, get a Dinosaur aftermarket board...much better than the OEM board.
Ken