Hi James Wilson,
Try the steps here: Create, edit, and delete playlists in Music on Mac - Apple Support
Once you click and open the link, please read through and complete the steps in the section titled "Add items to a playlist."
1) In the Music app
on your Mac, click Listen Now, Browse, or any option below Library in the sidebar to see songs you want to add to your playlist.
2) To add items to a playlist, do any of the following:
*From anywhere in your music library, drag an item to a playlist in the sidebar.

*Control-click an item, choose Add to Playlist, then choose a playlist (or use the Touch Bar).
Tip: To add songs to a playlist without adding them to your library, choose Music > Settings, click Advanced, then deselect “Add songs to Library when adding to playlists.”
We recommend the steps above because they will show you add to add a song to a playlist.
If you’re still experiencing the same issue, reach back out to the community for more help.
Thanks for using the Apple Support Communities.
Cheers!