Screen-Sharing “no network selected for sharing” problem on Ubuntu
Gnome not allowing Screen Sharing when no network connected.
Gnome gets the information from the NetworkManager. And it is not reporting any network because it is not set to manage those connections. This had to be fixed.
This is what was in /var/log/syslog
gnome-control-c: Failed to enable service vino-server: GDBus.Error:org.freedesktop.DBus.Error.InvalidArgs: Sharing cannot be enabled on this network, status is '0'
To fix: change managed to true in /etc/NetworkManager/NetworkManager.conf
sudo touch /etc/NetworkManager/conf.d/10-globally-managed-devices.conf
Then reboot, and check whether now there are networks in the "Networks" field on your screenshot. The "Screen Sharing" button now should allow to be set to "On"