Thanks, not everyone is able to recognize the standard update dialog, so it's good to confirm.
This may not be correct for all versions of macOS, but try deleting this file:
~/Library/Preferences/com.apple.preferences.softwareupdate.plist
While in the Finder, press "command shift G", paste the above line into the window that opens and press Return. It will highlight the file for deletion. Delete the file, restart your Mac, then go to System Preferences and run Software Update again and see what if anything comes up. Install anything it may shows. Then see if that stops that constant incorrect nag.
If not, post back and perhaps someone will have additional suggestions.
Hope this helps.