Macos – “You are not in the sudoers file” error after running sudo command

Ubuntu locks the root password by default, so you cannot login as root or su into it. Normally your userid would belong to the admin group and the admin group has sudo permissions granted via /etc/sudoers.

Find out which id you're logged in as using the id command. This is obviously the wrong "shawn" id as you don't have sudo permissions.

Now, reboot the system. At the GRUB screen, press Esc to boot into recovery mode. In recovery mode, you are root. Delete the unwanted id (userdel) or just modify it to a different name (usermod). Reboot. You should be able to log into the correct id now (id command should show a different uid). If this doesn't work, just reboot again into recovery mode. Delete the other id too. Create a new id and add it to the admin group.

Edit: I just realized that SU had a very similar question in the sidebar, and following that led me to this answer that has a meticulously detailed write-up on booting into recovery mode (and editing the /etc/sudoers file).

Say your admin account name is admin, then:

su admin

Then you are the admin user...

.....

And Gordon Davisson has a good suggestion. When you're a user with admin rights, you can use sudo to get root rights even if there is no root user. And you can use "sudo john" to become user John without knowing his password.