This is from another answer I posted recently. You may not need the Login screen section.

Terminal CLI method

To prevent screen from turning off you need two settings, one under battery power the other when plugged into wall outlet (A/C).

For battery timeout:

gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-battery-timeout <time_in_seconds>

For AC timeout:

gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout <time_in_seconds>

So for both Battery and A/C set the time to 0 (never).

For Login screen:

gsettings set org.gnome.desktop.screensaver idle-activation-enabled false

Disable dimming screen when idle:

In Unix & Linux someone complained when on battery screen dims every 20 seconds and wants to turn that feature off: