By default Ubuntu will lock your desktop automatically after screen blank off, below steps will show you how to disable it.

Method 1 : Via GUI

1.On your top left of your screen , click the Activities button , then search “privacy” and open it

2.On the right part ,click the “screen lock” , then you can disable the automatic screen lock

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Methond 2 : Via commands

1.Open a terminal, run below command to check the status of automatic screen lock , true means enabled ,false means disabled

gsettings get org.gnome.desktop.screensaver lock-enabled

2.Then you can use below command to disable it

gsettings set org.gnome.desktop.screensaver lock-enabled false

3.In case of you want to enable it again , you can run below command

gsettings set org.gnome.desktop.screensaver lock-enabled true