How can I enable the Debug Menu in the App Store for Sierra? The
defaults command no longer seem to turn it on.
As far as I know, the debug menu is removed from Sierra. Try to clear Mac App Store caches to fix some downloading issues.
Launch Terminal and type the following command:
open `getconf DARWIN_USER_CACHE_DIR`/com.apple.appstore
Move all the files and folders out of com.apple.appstore. Then relaunch Mac App Store. You can delete all the files and folders if Mac App Store get back to work.