"Why 'notify-send' is not found"You seem to be assuming that the package will be called notify-send. Not all packages are named for the commands inside it.
When I go http://packages.trisquel.info/ and use the search option to find "packages that contain files named like this" I find that /usr/bin/notify-send is in the package called libnotify-bin. So that is the package name. Ta Da.
