Don't you have ubuntu tweak installed? If so, don't you have set under Tweaks > Session indicator "Disable lock screen?" If unsetting it corrects your problem, could you set this to invalid? Otherwise, set it to confirmed.
** Changed in: gnome-control-center (Ubuntu) Status: New => Incomplete -- You received this bug notification because you are a member of Ubuntu Desktop Bugs, which is subscribed to gnome-control-center in Ubuntu. https://bugs.launchpad.net/bugs/882180 Title: Cannot set keyboard shortcut for "Lock Screen" To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/882180/+subscriptions -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs