Software Update Problem
Many schools and corporations point Macs at internal software update servers to control which updates arrive and when they get enabled. 


Contact the IT department or support if you have an institutional Mac. They might fix the server or point your Mac to another source for updates. 
You can sever that connection from the following post. 


Using Software Update Server



You can revert your settings by running the following Terminal command:

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL


Alternatively, you can delete any profiles listed in the system preferences app. Be sure you have a backup before deleting or changing these settings. You might need to revert things if in fact the settings were needed to prevent some updates.