We presently have the same problem. Seems this might be a common issue with this model of LG Washing machine
I will advise that you take this to a LG service center, especially if it is still covered by a warranty.
If for any reason you cannot do this first, then you can try and do a simple check yourself. You can try and remove the back panel but remove the drain pipe first. The back panel is held in place by a single screw around the middle and clips at different points.
Once opened, you will see a long flat and thin plastic connecting the drain control knob at the top with the valve controlling the drain point. Disconnect it easily (it is clipped together) and remove the valve by unscrewing it. Try and check if there is anything blocking the flow of water there.
Clean up and then fix back. Test by adding water to the machine and opening the drain using the drain control.
If this does not fix the issue, then you need to take this to one of LG service centers to fix.