The Spotify plugin uses a daemon called Spop to actually playback Spotify tracks. For some reason it isn’t starting on your system. The below commands are trying to start it as a Linux service and it’s not being found. I have no idea what is behind the problem though.
Jan 20 19:18:33 volumio volumio[966]: info: CoreCommandRouter::executeOnPlugin: spop , saveSpotifyAccount
Jan 20 19:18:45 volumio volumio[966]: info: Following Redirect to: http://volumio.github.io/volumio-plugins/plugins/volumio/armhf/plugins.json
Jan 20 19:18:50 volumio volumio[966]: info: Killing SpopD daemon
Jan 20 19:18:50 volumio sudo[5708]: volumio : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/usr/bin/killall spopd
Jan 20 19:18:50 volumio sudo[5708]: pam_unix(sudo:session): session opened for user root by (uid=0)
Jan 20 19:18:50 volumio sudo[5708]: pam_unix(sudo:session): session closed for user root
Jan 20 19:18:50 volumio volumio[966]: info: Cannot kill spop Daemon
Jan 20 19:18:50 volumio volumio[966]: info: CoreCommandRouter::volumioRemoveToBrowseSourcesSpotify
Jan 20 19:18:50 volumio volumio[966]: info: CoreCommandRouter::executeOnPlugin: my_music , getDisabledSources
Jan 20 19:18:50 volumio volumio[966]: info: Disabling plugin spop
Jan 20 19:18:50 volumio volumio[966]: info: Done.
Jan 20 19:18:53 volumio volumio[966]: info: Enabling plugin spop
Jan 20 19:18:53 volumio volumio[966]: info: Loading plugin "spop"...
Jan 20 19:18:53 volumio volumio[966]: info: PLUGIN START: spop
Jan 20 19:18:53 volumio sudo[5724]: volumio : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/systemctl start spop.service
Jan 20 19:18:53 volumio sudo[5724]: pam_unix(sudo:session): session opened for user root by (uid=0)
Jan 20 19:18:53 volumio sudo[5724]: pam_unix(sudo:session): session closed for user root
Jan 20 19:18:53 volumio volumio[966]: info: The following error occurred while starting SPOPD: Error: Command failed: /usr/bin/sudo /bin/systemctl start spop.service
Jan 20 19:18:53 volumio volumio[966]: Failed to start spop.service: Unit spop.service failed to load: No such file or directory.
Jan 20 19:18:53 volumio volumio[966]: info: Error: Error