Sierra - App Store Debug Menu
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.