Answer by Sajid Ali for Sudoers file syntax error on WSL

Close the WSL terminal, open Windows command prompt and make the default WSL user as root using the below command

ubuntu2004 config --default-user root

Open a new WSL terminal, you will be logged in as root.Type

nano /etc/sudoers

Now you can edit the sudoers file. Cheers!

Extra: ubuntu1804 config --default-user root for Ubuntu 18.04 and above or ubuntu config --default-user root for versions below.