hdiutil convert /tmp/ElCapitan.dmg -format UDTO -o ~/Desktop/ElCapitan_10.11.6.iso
Apple does not provide a native ISO file. If you need an ISO for a virtual machine (like VMware or VirtualBox), you must convert the official installer: How to Download OS X El Capitan 10.11 (DMG, ISO, APP) Mac Os X El Capitan 10.11 6 Iso Download
If you specifically need an ISO for software like VirtualBox or VMware, you must use a sequence of Terminal commands to create a blank disk image, mount it, use the createinstallmedia hdiutil convert /tmp/ElCapitan
To install Mac OS X El Capitan 10.11.6 on your Mac or virtual machine: Mac Os X El Capitan 10.11 6 Iso Download