sudo relies on a database of allowed users and actions in
/etc/sudoers. This error occurs because this file does not exist, so
sudo doesn't know who is authorized to do what.
You need to recreate
/etc/sudoers with a valid configuration. Assuming you want to authorize users in group "sudo" to elevate to root, this would work (as root):
cat > /etc/sudoers << 'EOF'
root ALL=(ALL) ALL
%sudo ALL=(ALL) ALL
If you can't get a root shell any other way, or your root password is disabled, perhaps the easiest way to do this is to reboot, set
init to your shell, then reboot again. For example, with GRUB 2, hit e at the bootloader to edit your menu entry, add
init=/bin/bash to your kernel parameters, press F10 to boot, and run the above command from the shell that appears.