Automatic suspend - "Computer will suspend very soon because of inactivity" - how can I disable this?

I'm running Ubuntu 17.10 on VM.

In the guest I opened GNOME terminal, launched Aptitude with sudo aptitude, then switched to the host machine to do some tasks.
When I returned to the Ubuntu guest it was showing this notification on lock screen:

Power
1 new notification

and after unlocking:

Automatic suspend
Computer will suspend very soon because of inactivity.

Just after unlocking Aptitude says that it can't download packages because the system is offline.

How can I disable this behavior?

The solution is to disable automatic suspend:

1. Open GNOME Control Center, go to Power tab (or simply gnome-control-center power)
2. In Suspend & Power Button set Automatic suspend, to Off when Plugged In.

This will change settings from

$ dconf dump / | grep -i "suspend\|sleep"
sleep-inactive-ac-timeout=900
sleep-inactive-ac-type='suspend' # <
sleep-inactive-battery-timeout=900

$ gsettings list-recursively | grep -i "suspend\|sleep"
org.gnome.desktop.screensaver ubuntu-lock-on-suspend true
org.gnome.settings-daemon.plugins.power button-power 'suspend'
org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout 900
org.gnome.settings-daemon.plugins.power power-button-action 'suspend'
org.gnome.settings-daemon.plugins.power lid-close-suspend-with-external-monitor false
org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type 'suspend' # <
org.gnome.settings-daemon.plugins.power button-suspend 'suspend'
org.gnome.settings-daemon.plugins.power button-sleep 'hibernate'
org.gnome.settings-daemon.plugins.power sleep-inactive-battery-timeout 900
org.gnome.settings-daemon.plugins.power lid-close-ac-action 'suspend'
org.gnome.settings-daemon.plugins.power sleep-inactive-battery-type 'suspend'
org.gnome.settings-daemon.plugins.power lid-close-battery-action 'suspend'

to

$ dconf dump / | grep -i "suspend\|sleep"
sleep-inactive-ac-timeout=900
sleep-inactive-ac-type='nothing' # <
sleep-inactive-battery-timeout=900

$ gsettings list-recursively | grep -i "suspend\|sleep"
org.gnome.desktop.screensaver ubuntu-lock-on-suspend true
org.gnome.settings-daemon.plugins.power button-power 'suspend'
org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout 900
org.gnome.settings-daemon.plugins.power power-button-action 'suspend'
org.gnome.settings-daemon.plugins.power lid-close-suspend-with-external-monitor false
org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type 'nothing' # <
org.gnome.settings-daemon.plugins.power button-suspend 'suspend'
org.gnome.settings-daemon.plugins.power button-sleep 'hibernate'
org.gnome.settings-daemon.plugins.power sleep-inactive-battery-timeout 900
org.gnome.settings-daemon.plugins.power lid-close-ac-action 'suspend'
org.gnome.settings-daemon.plugins.power sleep-inactive-battery-type 'suspend'
org.gnome.settings-daemon.plugins.power lid-close-battery-action 'suspend'

Summary:

Setting->Power : Suspend: "off"

Or open a terminal, and execute the command:

gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type 'nothing'

Detailed:

Use the setting dialog "Power" and set the automatic suspension to "OFF". Sometimes the setting is not available in the setting-dialog; it is simply missing. (e.g. Only the screen-timeout is shown.)

Please use a terminal instead then.

Check the current setting with the command:

gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type

it should reply:

'suspend'

Please check if the key is writable:

gsettings writable org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type

it should reply:

true

Check the range:

gsettings range org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type

it should reply:

enum
'blank'
'suspend'
'shutdown'
'hibernate'
'interactive'
'nothing'
'logout'

And set the value (with single, double or no quote on "nothing") to your choice:

gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type 'nothing'

Check afterwards, if the setting has changed:

gsettings get org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type

it should show your changes:

'nothing'

Hint: Please avoid to use sudo or a root shell, this will prohibit the access to the local settings.

Just to add that given the myriad versions of the GUI control centre that appear to be floating around (mine looks nothing like the one upthread) I found the easiest way to make this change was:

gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-type "nothing"