You may have an old copy of the song file (that has DRM), even though you purchased the whole album later.
If you purchased the complete album (in 2014), including that song (originally purchased in 2006), you may want to try downloading it again. Just in case, make a copy of the current song file. You can do this by dragging the song from the iTunes window to the desktop. The song file should appear on the desktop.
NOTE: When you do the next part, if this song is currently on any playlists, you'll have to add it back again. Also, its rating and play count will be set back to zero.
Select the song in your iTunes music library. Right-click it and select Delete. Confirm action and select option to send song file to the trash. The song should no longer appear in your iTunes music library.
Go to the iTunes Store main screen. If you are not already, sign in using your Apple ID (the one used when you bought "the whole album"). Along right side of window, find and click Purchased. On the Purchased screen, you can download previous purchases again. Set it to show Music that is Not in My Library. Find that song and click its "download cloud" to download it.
Back in your music library, that song should be back. See if it now syncs to the iPod without that error message about authentication.