This error happens if a distribution does not have: gir1.2-nm-1.0 installed.
Please install gir1.2-nm-1.0 from the command line with:
sudo apt install gir1.2-nm-1.0
Also make sure you also have the following packages installed: network-manager-openvpn-gnome and network-manager-openvpn
After that try to run Tunnel again.