After upgrading to ubuntu 20.04 Lockscreen not working

I was having EXACTLY the same problem. It turns out I have several display managers installed, and I was using lightdm instead of gdm3.

You can confirm you are using lightdm if this command works:

$ dm-tool lock

(But even if that doesn't work you may be using another display manager.)

I got the screen blanking to work properly by configuring gdm3.

Open a terminal and issue this command:

$ sudo dpkg-reconfigure gdm3

Select gdm3 in the list on the terminal, then reboot your system.

I have the same gsettings get org.gnome.desktop.lockdown disable-lock-screen config and Super+L works

Could you check Settings > Keyboard Shortcuts > search(Super+L), this one should be assigned to lock the screen.

The lock is a feature of gnome-session. In your research maybe swap "ubuntu" for "gnome" or "gnome3.36.2" with a bit of luck you have the same as me ;)