Music not showing up when trying to sync to iPod Nano 6th generation

I'm trying to sync music from my iMac Pro running OS X Catalina to an old iPod Nano (6th generation). The Nano shows up in finder. I click on it, and choose "Sync music onto iPod". Then I choose "Selected artists, albums, genres, and playlists."

Problem is, almost nothing appears under the Artists, Albums, Genres, and Playlists tabs—just a fraction of the music that I can see in my library in the Music app. Why is this, and how can I get my entire library to appear there?