Case: After creating a file in /etc/sudoers.d/ and adding command aliases and a user privilege specification, the file became inaccessible and sudo-ing no longer worked on Ubuntu.
Solution
Use pkexec instead sudo when accessing the file.
“pkexec allows an authorized user to execute PROGRAM as another user.”
For example, use:
pkexec visudo -f /etc/sudoers
instead of:
sudo visudo -f /etc/sudoers
Same goes for visudo -f /etc/sudoers.d/some_file.