Install xautolock by entering the following commands in the terminal:
sudo apt update sudo apt install xautolock
Description:
Program launcher for idle X sessions
Xautolock monitors input devices under the X Window System, and launches a program of your choice if there is no activity after a user-configurable period of time. You can use this to automatically start up a screen locker if you have left your computer unattended for some period of time. The program launched need not be a screen locker such as xlock.
Version: 1:2.2-5.1
Section: universe/x11