If you are stuck in the Jamming project in React course because your application won’t save your tracks in your spotify account but created your playlist empty well then that’s because spotify has changed the playlist endpoint. The endpoint given in step 94 has been changed by spotify.
try - POST https://api.spotify.com/v1/playlists/{playlist_id}/tracks
instead of - POST https://api.spotify.com/v1/users/{user_id}/playlists/{playlist_id}/tracks
when you are adding tracks to the playlist. see this blog post.
Hope this helps.