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 SPOTIFY source icon in the BROWSE PAGE
Do the same steps to install SPOTIFY CONNECT plugin, in order to stream Spotify source lossless directly from the Spotify APP to Volumio.