Ubuntu 20.04 not work lock screen after update

I had this problem as well, and after not finding a solution on S.O. / AskUbuntu, I went over to Luanchpad to poke around for related bugs or questions.

It seems this issue is specific to LightDM, and is perhaps part of a bigger security issue, and switching to GnomeDM (gdm) fixes the issue. Here are the steps I followed:

Install GDM if not already installed:

sudo apt-get install gdm3

Switch from LightDM to GDM3:

sudo systemctl status display-manager

Reset/reinstall some stuff:

sudo dpkg-reconfigure lightdm
sudo apt install --reinstall ubuntu-session

Reboot, and you should be good. Note that I don't know if switching back from GDM to LightDM will maintain the fix. From the links below it sounds like it may be part of a broader security issue and the Super+L keyboard shortcut is disabled for a reason.

More links to background:
https://answers.launchpad.net/ubuntu/+question/693197
Can't lock screen with shortcut on Ubuntu 20.04 gnome
https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1806961