The F21 and F33 are both related to the drain pump. You have already cleaned the drain filter you say so the problem will most likely be with the drain pump or the central control.
When the washer is not draining you don't mention if the drain pump seems to be running or humming or completely dead.
With the washer unplugged you can check the connections at the pump and the central control. You remove the top, two screws in the rear, to access the central control and you already know how to access the pump.
If you have access to a volt/ohm meter and are comfortable using it then check resistance across the two pump terminals. An open winding will mean the 8540024 pump is bad.
If the pump is okay then the W10205849 main control is most likely at fault.
The pump is the more likely of the two and if you have no meter or don't know how to use one then I'd recommend you start with the pump. It's the cheapest and you already know how to access it.
I hope this information is helpful to you but submit another question with any additional details for further assistance.