Answer by рüффп for Sudoers file syntax error on WSL

On my side I just edit the sudoers file from Notepad++ in Windows:

Open the file from <LinuxSubSystem_Home>\Ubuntu18.04\rootfs\etc\sudoers

I corrected the bad entry and saved it.

My WSL was starting fine after the correction.

Just in case, here is the default sudoers file I had

## This file MUST be edited with the 'visudo' command as root.## Please consider adding local content in /etc/sudoers.d/ instead of# directly modifying this file.## See the man page for details on how to write a sudoers file.#Defaults    env_resetDefaults    mail_badpassDefaults    secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"# Host alias specification# User alias specification# Cmnd alias specification%sudo ALL=NOPASSWD: /sbin/start-stop-daemon%sudo ALL=NOPASSWD: /etc/init.d/ssh# User privilege specificationroot    ALL=(ALL:ALL) ALL# Members of the admin group may gain root privileges%admin ALL=(ALL) ALL# Allow members of group sudo to execute any command%sudo   ALL=(ALL:ALL) ALL# See sudoers(5) for more information on "#include" directives:#includedir /etc/sudoers.d

It's possible that some entries have been added by "apt install" command.