Let's say something like this is being returned when attempting to issue a command using sudo.

sudo: parse error in /etc/sudoers near line 23
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

Naughty Naughty! This means someone editted a sudo file without using visudo. The visudo command would not let you save a change to a sudo file that would cause a sudo file to be misconfigured. Sudo files should always be editted using visudo.

OK - now that the lecture is done, how to fix this. Fortunately, the pkexec almost always can be used to fix a broken sudo file. For example, if the main sudo.conf file is misconfigured, you can use the following command to find and fix the misconfiguration.

pkexec visudo

Or, if a child sudo file is misconfigured, then the -f flag can be used to fix the misconfigured child sudo file.

pkexec visudo -f /etc/sudoers.d/my.file