software

Check here: Use the Mac operating system that came with your Mac, or a compatible newer version - Apple Support

click the OS Version you wish to have and see if your Mac requirements will run that version

That was indeed the last build of Mevericks...

13F1911July 18, 2016 About the security content of Security Update 2016-004 Mavericks Security Update 2016-004 Mavericks

See if you can upgrade to 10.12, 10.13, or 10.14, see sys requirements...

https://support.apple.com/kb/sp742?locale=en_US

https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&mt=12

https://support.apple.com/kb/sp765?locale=en_US

https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12

https://support.apple.com/kb/SP777?locale=en_US

https://itunes.apple.com/us/app/macos-mojave/id1398502828?mt=12

https://support.apple.com/en-us/HT206886

macappstores://itunes.apple.com/app/id1246284741

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?ls=1&mt=12

CatalogURL for macOS 10.13 (High Sierra)...

https://pikeralpha.wordpress.com/2017/06/06/catalogurl-for-macos-10-13-high-sierra/

Sometimes the system does not allow downloading high sierra from the app store.

Therefore, I offer you a terminal command that I have currently tried and that allows you to download MacOS high sierra from the App store.

open macappstores://apps.apple.com/us/app/macos-high-sierra/

If you go to the High Sierra page and it says that it is already installed, it might actually mean you have the Install macOS High Sierra.app in your Applications folder, or somewhere on your Mac.

The App Store Updates page only lets you know about updates available for the OS you are running. going from

one macOS to a newer macOS is called an upgrade and they are never in the App Store Updates page.

They can be infuriating.