I just updated Audacity, and I am trying to make a mono recording into a stereo track. I am following all the normal steps (select track → edit → duplicate) except there is just one problem. Even though I do have both tracks selected, the ‘Make Stereo Track’ option in the drop down menu (on the top track, which is the original and not the duplicate) is greyed out.
I’m using Windows 10, Audacity 3.0.3.
Thank you for taking the time to read and help me out. 
The tracks don’t need to be selected.
I’m not able to reproduce the problem. Are you able to reproduce it in a new project? If so, please provide steps to reproduce.
Hi Steve, I just did reproduce it in a new project and I am still having the same problem. 
I went to File → New
I imported my mono track. I duplicated it, and I still am getting ‘make stereo track’ greyed out.
Another way to try it -
Tracks → Add New → Stereo Track
Select the mono track, then copy & paste into the stereo track.
Delete the original mono track.
P.S.
This is a true-mono track, right? You don’t have a stereo track with silence on one side?
Another way -
File - Import audio, twice.
Then you should be able to Make Stereo Track.
Perhaps try “Tools menu > Reset Configuration” and see if that clears the problem.
I tried that, no luck. Thank you for your help.
Thank you so much, this actually worked. I believe it is a true mono track, since I used a Zoom recorder with an XLR microphone that only goes into one side, not both. However, I did have to split the track to be able to copy and paste it into the new track. So perhaps it is not?
I’m still new to this!
Could you post a screenshot showing the greyed out menu item and the whole of the Audacity window? We “may” be able to spot the problem if we can see it.
I think we may have missed a cue from Steve at the top.
If you have two mono tracks one above the other, you only select the top one and choose Make Stereo Track.
It will automatically select that track for Left and the next track down for Right in a stereo show.
If you select the bottom track, Make Stereo Track will be gray.
Koz
I had this issue today after splitting a stereo track and making edits on one of the tracks.
I was able to resolve it by adding a new mono track below, checking to see if it was no longer greyed out on the second track from the top, and then deleting it afterwards.