Lock screen shortcut is super + L.
But for your issue it seems the setting is disabled.
It's usually a dconf/gsettings setting so look at what this tells you
Code:
gsettings get org.gnome.desktop.lockdown disable-lock-screen
should be set to false.
You can try a reset of that line
Code:
gsettings reset org.gnome.desktop.lockdown disable-lock-screen
which should put it back to the system defaults for the lock screen.
If the defaults just rests it back to true then try manually doig it
Code:
gsettings set org.gnome.desktop.lockdown disable-lock-screen false
Alternatively, this can be done graphically in dconf-editor which needs to be installed.
(Just follow the same path as shown > org > gnome > desktop > lockdown > profit)
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