Librespot doesn’t support Facebook OAuth, so you need to:
At the top right of the desktop app, select your username, and go to Account.
You should be on the “Account overview” page.
You will see your Spotify “Username” there, which is a long string of randomized characters. Save that.
Now go to the “Set device password” page and set a password for your account.
Now log out of the Spotify desktop app, and log in without using Facebook, by using the (long random characters) username & password you just set.
Now the desktop app will send normal Spotify access tokens over Spotify Connect and it should work fine!
I don’t know why I couldn’t find this information anywhere on Volumio’s site and had to figure it out myself by looking at librespot’s open issues 