Spotify does not support music streaming on iPod Nano, Classic, or Shuffle. I‘ve read a post in the Spotify Community, saying that some old generations of iPod Touch can install the old Spotify app which stopped updating. Not sure whether this method is available for iPod Nano. Apologies that I cannot find that post now.
https://www.audfree.com/spotify/sync-spotify-to-ipod.html This article here might be helpful to you. I use the Audfree music converter for Mac to download Spotify songs and then sync them to my iPod Classic.