This is a PX4 Firmware bug as mentioned here and here. The python package ‘empy’ used by WSL must be downgraded to version 3.3.4. You can follow the below instructions. 
- Launch WSL where PX4 is cloned.
- Run the below commands in WSL.
                        pip uninstall em
            pip uninstall empy
            pip install empy==3.3.4
Launch the PX4 Hardware setup screen and try building the firmware now.

