Ubuntu – Cannot turn on sharing on ubuntu 18.04

You can turn it back on by clicking on your monitor number in the Display menu and selecting the monitor that is turned back off. Screengrab below: display monitor selection

Each monitor has a number. If you go to Settings > Devices > Screen Display and find the monitor number that is your primary monitor. Click on the number and it will show you a dropdown menu of all available monitors. Select the one you turned off and turn it back on.

This happened to me, here is how I corrected it.

My network showed "wired-unmanaged"; to correct this, I had to do the following:

Note: If you're interested in a set of commands to copy/paste into your terminal, I'd encourage you to review these instructions first, before running the commands listed at the end of this answer.

• cd /etc/NetworkManager
• save NetworkManager.conf to NetworkManager.orig (as a backup)

• sudo vi NetworkManager.conf

  Change managed=false to managed=true

  New file looks like this:

  [main]
      plugins=ifupdown,keyfile
  [ifupdown]
      managed=true
  [device]
      wifi.scan-rand-mac-address=no
  

• sudo service network-manager restart
• cd /etc/netplan

• sudo vi 50-cloud-init.yaml

  Add this line just below network:

  renderer: NetworkManager

  New file looks similar to this (ensure the renderer line is indented as shown):

  network:
      renderer: NetworkManager
      ethernets:
          enp3s0:
              addresses: []
              dhcp4: true
  version: 2
  

• save
• sudo netplan apply
• I had then to restart the computer for this to be effective.
• After restart the network will now show "wired-connected"
• Then you can go to Settings » Sharing » Screen Sharing
  • You should now be able to toggle Screen Sharing to ON
  • Under Networks (bottom of dialog), toggle those ON as well

Commands

If all you're looking for is some terminal commands, and you haven't made any manual changes to the files mentioned in the instructions above, running the commands below should take care of the file modifications:

cd /etc/NetworkManager
sudo cp NetworkManager.conf NetworkManager.orig
# Change NetworkManager.conf: 'managed=false' to 'managed=true'
sudo sed -i 's/managed=false/managed=true/g' NetworkManager.conf
sudo service network-manager restart
cd /etc/netplan
# Ensure networks are managed by NetworkManager
# Whitespace in this command is important!
sudo sed -i '/^network:/a \    renderer: NetworkManager' 50-cloud-init.yaml
sudo netplan apply
# Restart your machine, or uncomment one of the lines below:
#sudo shutdown -r now
#gnome-session-quit --reboot

After you run these commands, refer to the final step above to enable screen sharing.