Nautilus show folders before files does not work in Ubuntu 18.04 LTS

You can change this from the Nautilus preferences. Open the context Menu from the top-right of a Nautilus Window:

Tick 'Sort folders before files'.

If that doesn't work you can edit the Dconf file directly, the easiest way is using the Dconf Editor: Install Dconf Editor (sudo apt install dconf-editor), open it and then navigate to:

org → gtk → settings → file-chooser → sort-directories-first and set it to True.

Alternatively, run this command in the terminal:

dconf write /org/gtk/settings/file-chooser/sort-directories-first true

It is also possible to use the pre-installed gsettings command line utility for that:

gsettings set org.gtk.Settings.FileChooser sort-directories-first true

The current setting can be checked like this:

gsettings get org.gtk.Settings.FileChooser sort-directories-first