According to the Picard documentation %albumsort% is not created by Picard and you need to have a script to create it with $set(albumsort,$swapprefix(%album%)).

I propose that (in the absence of this value being provided by MusicBrainz - which does not currently hold sorted versions of the Album name) Picard automatically creates this variable/tag by running the swapprefix function against the album title.