Install arctica-greeter-guest-session by entering the following commands in the terminal:
sudo apt update sudo apt install arctica-greeter-guest-session
LightDM Arctica Greeter - Guest Session Support
A greeter shell for the LightDM login manager. Arctica Greeter can be used as local display manager as well as thin client login manager. . This package adds guest account support to the Arctica Greeter login manager.