App Store is stuck downloading Software Update
The simplest might be to just bypass the App Store altogether &amp; get the update manually - this will only work if you are already on El Capitan, any version, which the App Store picture would indicate you are.

Just fetch the 10.11.4 Combo Updater directly from Apple. This will bypass the App Store's delta updater, &amp; also likely clean up whatever is causing the issue in the first place, by re-applying all updates since 10.11.0