Greetings and How do I kill the x server

[Jim]

I have a problem killing the x server.  I have Kubuntu 18 installed.  
I've gone into system settings / input devices / keyboard and enabled 
the option to kill the x server using control alt backspace, but it 
doesn't work.  I created another user account and it works if I'm logged 
in as the other user.  I know I could nuke all this account's settings 
and start from scratch, but I have this account configured like I like 
except for the ability to kill the x server.  Does anyone have any idea 
what file needs to be edited or deleted, or is there something else?  Thanks