The System Preferences> Software Update mechanism did not come into being until macOS Mojave (10.14)
The Updates page in the App Store only informs you of updates available for the OS you are currently running,
as you are on 10.11.6 El Capitan then you are probably up to date on that.
Open Safari and click on the link offered by Dialabrain.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says 'Open' then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS High Sierra.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.