“Stereo track to mono” is grey/not active. What can I do then???
When I export it to wav or mp3, it is made into 1 track - so my problem is solved 
I just cannot understand why the first solution is not possible
Tracks > Stereo Track to Mono requires a track that says “Stereo” to left of the blue waves.
Stereo Track to Mono is not active if you have already split the stereo track into left and right channels. You could do Mix and Render instead in that case (and you should do that instead of “Make Stereo Track” if you have added envelope points or changed the gain or pan sliders on the tracks).
Gale