The Main PCB does not detect that any or not enough water has drained out of the tub based off the pressure sensor frequency. 

Is the drain filter and pump housing clean?

Is the drain hose kinked, twisted, or blocked with debris?
Has the drain hose been installed correctly?  No more than 4 inches into the standpipe.

Is there a laundry item stuck in the outer tub drain hole or bellows to the pump housing?

Is the air chamber or pressure sensor hose blocked?

Check the harness connection at the drain pump and Main PCB.

Is the harness loose or disconnected?

Is the impeller of the pump damaged?
Measure the resistance of the pump motor windings.

The 24Vdc BLDC Drain Pump should measure approximately 1.5 ~ 4 ohms. Check gray 3 pin connector white wire, blue wire & skyblue wire.

DC Motor Voltage Check: The Drain Pump motor is a 24Vdc Brushless Motor using pulse width modulation dc voltage but can not be measured using a multi-meter set to DC voltage. You will have to use a multi-meter set to AC voltage. The multi-meter needs to be a higher quality meter capable to read high frequency inverter pulses to get any meaningful voltage readings.

You can use the test mode to activate the Drain Pump motor and test the voltage. The voltage you should read is around 11-12Vac.

Note: The unit you are working on may have different wire colors please see your wiring diagram for exact colors of the unit you are working on.

The 120 Vac Drain Pump should measure 10 ~ 20 ohms. Check white 6 pin connector gray to blue.

Activate the drain pump in test mode.
Is proper voltage present to the pump? 24Vdc or 120Vac depending on specific model.
If not, check voltage at the Main board. If still no voltage, replace the Main board.
If proper voltage is present at the Main board, suspect a harness issue. You can test the continuity of the wires to confirm a bad harness.