update OS

The App Store Updates page only informs you of updates available for the OS you are currently running.

As you are running an older system then you are probably up to date on that system.

If you want to install a newer OS that is called an Upgrade. 

The model and year of your Mac will determine which macOS you can upgrade to,

read the following links.

How to upgrade to macOS Mojave - Apple Support

How to upgrade to macOS High Sierra - Apple Support

How to upgrade to OS X El Capitan - Apple Support

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 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 you are considering upgrading to Catalina be warned that it does not support 32bit apps.

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/. Once the list has been created you can then

contact the developers of the apps to see if they provide 64bit updates. 

How to upgrade to macOS Catalina – Apple Support

If you follow my link to El Capitan in my last post that will take allow you to download the full installer for of El Capitan.

It sounds like you previously tried to upgrade to El Capitan using an update which will only install if you are already running

a previous version of El Capitan.

The full installer version will be 6.2GBs and will install the last version of El Capitan which is 10.11.6.

So try again,

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Download OS X El Capitan.

This downloads InstallMacOSX.dmg to your Downloads folder.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the .pkg to the Install OS X El Capitan.app which you will find in 

your Applications folder, it should be 6.2GBs in size.

To start there installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to

an external drive for safe keeping. It is also possible to create a bootable USB installer disk, using the Install OS X El Capitan.app

in the Applications folder, and the createinstallmedia command in the Terminal app. Read the instructions here,

How to create a bootable installer for macOS – Apple Support

Back up your data, check your applications for compatibility, and download El Capitan, High Sierra, Mojave, or Catalina.

(180224)

Sorry to jump on to your answer. I have an iMac and I am having the same issue. The App store only has Catalina available but it won't let me update. I have more than enough storage space to accommodate the update from Yosemite to Catalina but still no luck. Do you have any suggestions, please?