Cant upgrade from Sierra 10.12.6 to latest update

I have a mid 2014 MacBook Pro Retina, its currently running on mcOS Sierra 10.12.6. I can't find a way of upgrading it to the latest MacOS.

I need to download apps from the App Store but its not letting me as my OS needs to be upgrade.

"(null) can’t be installed on “Macintosh HD” because macOS version 10.14 or later is required."

I tried going to app store to find a upgrade but nothing shows up. I went into About This Mac, Software Update, which redirects me to the App Store, and no update shows up. Please Help