If you make use of the sudoers.d directory, make sure to have it included in /etc/sudoers.d/
Edit your sudoers file with visudo and add the right option :
Make sure to write this line as it is. The # sign is compulsory. Here, it does not stand for commenting a line.
Otherwise, you will get this error :
sudo: /etc/sudoers.d is not a regular file
sudo: parse error in /etc/sudoers near line 69
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin