Where can I download OS X El Capitan 10.11.1?

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

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

Installing a newer OS is called an upgrade.

Any updates you downloaded from Apple Support Downloads are not full installers so if you have tried to install

El Capitan update 10.11.2 you need to be running an earlier version of Mac OS X El Capitan before you can install

the update. If you have tried to install a Security Update for Mojave you need to be running Mojave in the first

place before that will install.

The link below will take you to an Apple article with information how to get the full installers for various OS's.

If you want El Capitan you will be guided to download the InstallMacOSX.dmg which once downloaded and processed

will allow you to install a full working version of Mac OS X El Capitan and that version will be 10.11.6, this is all you need.

If you want to install Mojave you can download that via the App Store, the installer app will download directly to your Applications folder as Install macOS Mojave.app and will install version 10.14.6.

As your mac can upgrade to the newer OS's like macOS Mojave 10.14 and macOS Catalina 10.15, 

I would recommend you upgrade to macOS High Sierra 10.13 first.

Read all instructions thoroughly and take notice of the bold text.

For Mac OS X El Capitan, macOS High Sierra, macOS Mojave or macOS Catalina,

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to the Download OS section and click on the OS you want to install.

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 via cable 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 the App Store says ‘Open’ then it is detecting a previous download of the install app

go to the Applications folder locate Install macOS ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you are considering upgrading to Catalina be warned 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/ then contact the developers of the software to see

if they are offering 64bit updates.

Well you do not " know all that " because you are trying to download what is NOT Full Installer. What is being download are only updates to already Fully Installed versions of Catalina or Mojave.

If you follow the instructions provided and use Safari to start the download, it will open a redirect and open the Apple Store where you can download the FULL INSTALLER.

Not possible as Apple Only Offered the last version for an ancient macOS.

Good luck following very detailed instruction.

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