Try these:
sudo apt-get remove gnome-screensaver
sudo apt-get install xscreensaver xscreensaver-gl-extra xscreensaver-data-extra
Then search for "Screensaver" in the menu and tweak its settings to your needs.
To add Xscreensaver to startup, open Startup Applications and add "xscreensaver -nosplash".
To get CTRL + ALT + L to lock the screen and start the screensaver, go to System Settings > Keyboard and on the "Shortcuts" tab, under "System", change the "Lock screen" keyboard shortcut from CTRL + ALT + L to something else, then under "Custom Shortcuts", click the "+" button to add a new custom shortcut, under "Name" enter "Xscreensaver" and under "Command" enter "/usr/bin/xscreensaver-command -lock", then click "Apply".
And finally, click next to the newly created shortcut and press CTRL + ALT + L to assign it to it (or use any other keyboard shortcut you want, but make sure it's not already assign to something else). Alternatively, you can run the following command instead of assigning a new keyboard shortcut for the old screensaver:
sudo ln -s /usr/bin/xscreensaver-command /usr/bin/gnome-screensaver-command
To revert the changes (go back to the black GNOME Screensaver screen), use the commands below:
sudo apt-get remove xscreensaver xscreensaver-gl-extra xscreensaver-data-extra
sudo rm /usr/bin/gnome-screensaver-command
sudo apt-get install gnome-screensaver
If you've used the alternative method to assign CTRL + ALT + L to Xscreensaver lock screen, open System Settings > Keyboard again, remove the custom Xscreensaver command and assign CTRL + ALT + L back to "Lock screen" (under "System").
Best Answer
You don't need to install 3rd party package
You can re-enable lock screen directly from the terminal.
To see if Lock screen is disabled:
If result is
truethen use: