Hi RileyHarder,
From the information you've provided, it would appear that the copy of the song you're attempting to sync was sourced from Apple Music. This could be a function of iCloud Music Library, or that even though you purchased the song it was not downloaded to your Mac.
The site Download previous iTunes Store purchases in Music on Mac outlines the specific steps to download purchased tracks as:
"Download previous purchases to an authorized computer
- In the Music app on your Mac, make sure you authorized your computer to play iTunes Store purchases.
- Click iTunes Store in the sidebar.
- If you don’t see iTunes Store in the sidebar, choose Music > Settings, click General, make sure iTunes Store is selected, then click OK.
- Click Purchased on the right side of the iTunes Store window.
- Purchases available for download are displayed. You can view all of your past purchases or just the ones not currently in your music library. Items already in your library aren’t available for download.
- You can view your purchases by song, artist, or album. If you view by album, click an album to see which songs are available to download.
- To search for an item, type its name or a keyword in the search field, then press the Return key.
- To download an item, click its Download button ."
Once downloaded, you can confirm that you have the purchased version in your Library. In the Music app for Mac, right-click or Control+click in the heading bars above your library to view categories. Enabling the "Cloud Status" option will show results such as Uploaded, Purchased, Matched, or Apple Music. Ensure the track has Purchased or Uploaded in order to sync to your iPod.
Regards.