Have you got xscreensaver installed? One way to find out is to open up a terminal window and run
xscreensaver-demo
Another way is to add a lock screen button to a panel, right click on it and choose "configure screensaver" or something like that. Make sure you have some selected, and that they start correctly when you try them out. Locking the screen uses the screensaver subsystem, so if that's not installed or not working, lock screen will fail.
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