Apple - Mac App Store update stuck (Installing - Calculating...)

In an act of desperation, I managed to fix the issue by:

• Deleting /Library/Updates

• Doing

  sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist RecommendedUpdates
  sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate.plist DidRegisterLocalUpdates
  

  in a Terminal session

• Restarting the Mac

The update is re-downloading now.

Interestingly, before I tested these steps, I found the update ready to install in /Library/Updates, however when I tried to install the PKG file, Installer claimed that another installation needed to finish (?). Deleting it and restarting fixed the problem.

Try deleting the cache located here:

~/Library/Caches/com.apple.SoftwareUpdate/

And the prefrences plist, here:

~/Library/Preferences/com.apple.SoftwareUpdate.plist

And start the application again. It should have flushed whatever updates that got stuck.

This happened to me after updating to the Developer beta of macOS Sierra. For me, the following solution helped.

Delete the downloaded updates:

sudo rm /Library/Updates

Kill the software update daemon:

sudo killall -9 softwareupdated

An additional note to analyze AppStore / softwareupdate problems is probably using the Console app or tail to have a look at /var/log/install.log.