No lock button is present in the drop down of the status bar in Ubuntu 18.04.
When I go to Settings> Privacy> Screen Lock, I can click Screen Lock to open the Screen Lock window. In the Screen Lock window "Automatic Screen Lock" is dimmed.
Resetting the desktop theme restored the lock button.
dconf reset -f /
OMG Ubuntu: The Secret Command to Reset Ubuntu Desktop to Default Settings
$ dconf reset -f /
or $ sudo service lightdm restart
For more key words related dconf
command run : $ man dconf