Hey RockRider007,
It appears that you're not able to sync a Genius Mix to your iPod nano. You receive a message that it needs to be enabled in iTunes, but both of your Macs are on macOS Big Sur and iTunes isn’t there any longer. We're glad to glad to provide assistance.
What happened to iTunes? - Apple Support -- Beginning with macOS Catalina, iTunes no longer manages all of your media. You'll use the Music app instead for your music.
Use Genius Shuffle or Genius Playlists in Music on Mac - Apple Support -- Also, beginning with macOS Catalina, you can only create and use a Genius Shuffle or a Genius Playlist. This page of the Apple Music User Guide explains how.
Feel free to get back to us with any questions or problems you may have.
All the best.