v1.2.9 is a very old release that doesn’t support extensions, the current version is v1.2.37 and I would strongly recommend updating otherwise a lot of extensions (especially mine) might simply not work.
v1.3 beta however should’ve worked, did you have any luck with other extensions? If no extension works for you I’d recommend checking this thread as there were reported instances where Aseprite “loses” its configuration folder.