VNC Screen Share to connect Mac to Ubuntu

dconf > org -> gnome -> desktop -> remote-access > enabled - uncheck

Here is how you do it:

First make sure Desktop Sharing is set up properly.
Download dconf-tools by typing in Terminal sudo apt-get install dconf-tools
Run dconf-Editor
Expand org
Expand gnome
Expand Desktop
Select Remote Access
Uncheck Require Encryption (don't click on Set to Default as it rechecks it)
Exit dconf-Editor

http://ubuntuhandbook.org/index.php/2016/07/remote-access-ubuntu-16-04/