The file is mono (both channels are identical).
As the "analyze" option in "Vocal Reduction and Isolation" will tell you:
http://manual.audacityteam.org/man/voca ... ation.html
Pan position: 0
The left and right channels are correlated by about 100 %. This means:
- The two channels are identical, i.e. dual mono.
The center can't be removed.
Any remaining difference may be caused by lossy encoding.
I other words, it's not going to work.
Separating vocals from that track is likely to require long and detailed editing with a program such as "Melodyne" (from $399 at time of writing)