Can't change my track to mono

Hi - I’m trying to reduce the vocals on a track to make a karaoke backing. I’ve been following a YouTube tutorial and I have successfully imported the audio track, then split the single track into two separate ones, then selected the second and inverted it. Next it says I must change the tracks back to mono but this is greyed out for me so not possible.

I’m on 10.11.5 Mac - it’s a track recorded in the 90’s (if that’s relevant?!)…

Which version of Audacity are you using? (Look in: “Audacity menu > About Audacity”).