Hi,
I noticed that you cannot transform a selection when the active cel is empty (sorry, I found this bug long time ago, but as I found a roundabout back then, I forgot to report it ^^U).
This was really really frustrating until I found a roundabout: If you move the selection (ie: pressing left cursor and then right cursor on your keyboard) then you can normally transform the selection.
P.S: I’m using Windows 7 ultimate 64 bits. Aseprite v1.2.16.3-x64 (but this error happens on previous versions too).