Using blueman-applet to connect Ubuntu 17.10 to a Bluetooth speaker as an audio sink raised the error:

Connection Failed: blueman.bluez.errors.DBusFailedError: Protocol Not available

This is resolved with:

sudo apt install pulseaudio-module-bluetooth alsa-base
pactl load-module module-bluetooth-discover
sudo alsa force-reload
sudo killall pulseaudio
pulseaudio &