For many legacy Mac Pros (Mid-2010, 2012) and 2011 MacBook Pros, 10.11.6 represents the last version of OS X that runs certain legacy drivers. Pro Tools HD users with older Digidesign interfaces? They stop at 10.11.6. Owners of the Canon scanner that never got a Sierra driver? They stop at 10.11.6.
is available from Apple as a disk image (.dmg) file. This is primarily intended for users whose Macs are compatible with El Capitan but cannot upgrade directly via the App Store. Apple Support Official Download and Installation Direct DMG Download : You can find the official link on the Apple Support: How to download and install macOS Mac Os X El Capitan 10.11.6 Installer Dmg Download
If you have a Mac that ever downloaded El Capitan in the past, you can use softwareupdate : For many legacy Mac Pros (Mid-2010, 2012) and
Using the Mac App Store (on compatible Macs) Owners of the Canon scanner that never got a Sierra driver