mixing stero down to mono when Voice cancelling

Using Windows 10
Using 2.2.1
The drop down menu after inverting for a voice cancellation is absent of the MONO option(s) (both L&R Tracks).
Also the Mix Stereo Down to Mono option under the tab up top - TRACKS is not active.
I have no way to do this with out that option.
Sean Reilly

Drop-down from the top track > Make Stereo Track. That should make the top two tracks into stereo.

Then Tracks > Mix > Mix Stereo down to Mono.

When you get done, you might try Effect > Vocal Remover which is all what you just did in one step, or Effect > Vocal Reduction and Isolation. This one is less likely to mess up the song than straight Split/Invert/Mix.