Unable to update to latest software

In Apple parlance, there is a tremendous difference between update and upgrade.

MacOS Mojave has been updated as far as it can go, as there are no Mojave releases beyond MacOS 10.14.6.

However, perhaps your Mac is eligible to be upgraded to MacOS Catalina. That would require a "purchase" (at no charge) from the App Store. Here is how to proceed.

How to upgrade to macOS Catalina - Apple Support

Check here. Is there some status problem with the server? (There is not here in the U.S., but perhaps there is in your area.)

Apple - Support - System Status

Okay. This is a long shot. This fixes a problem for those on Mojave who are getting an "up to date" error".

Run this command in the Terminal then reopen System Preferences > Software Update.

sudo softwareupdate --reset-ignored

Are you on an admin account or is your Mac managed? I don't know why you are getting the message…

"/etc/sudoers is owned by uid 501, should be 0"

Someone will be along that will have more info. FWIW, I am on an admin account, and my uid is 501.

See if it works while in Safe Mode.

Use safe mode to isolate issues with your Mac - Apple Support

Welcome.