Spotify is not a native integrated service inside Volumio, so you need to install an add-on plugin before activate your Spotify account.
Go to SETTINGS menu
Go to PLUGINS menu
Search for SPOTIFY between MUSIC SERVICES PLUGINS
Download and Install SPOTIFY PLUGIN
Go to INSTALLED PLUGINS and open SPOTIFY
Enter the credentials
DONE
You will find the streaming service access SPOTIFY source icon in the BROWSE PAGE of the User Interface