Can't load data from the Apple Software Update server
Please try this guide. At least it was useful for me. 

By default, the built-in application firewall for macOS automatically allows applications signed by Apple to receive incoming connections. If you've adjusted the settings for your application firewall, you might need to allow incoming connections. If the issue persists, reset the cache of accepted certificates:


Click your desktop to make sure that you're in Finder.
From the menu at the top of your computer screen, choose Go > Go to Folder.
In the box, enter /var/db/crls/.
Click Go.
Drag the files labeled crlcache.db and ocspcache.db to the Trash.
If prompted, enter your administrator password.
Restart your computer, then try to connect to the iTunes Store, iBooks Store, or App Store.