I am middleclicking (which is the current bind for sorting) and it will not sort chests, I am using 1.12.2 on a server

note that it works on singleplayer

As this is really old, my assumption here is that you've tried to use the mod on a server without the mod. If this is the case, this will not work, per the readme. You will need to use the mod whilst the server also has the mod.