If you are trying to download the OS X El Capitan installer to upgrade or reinstall on an older Mac, you may encounter broken links, "item unavailable" errors in the App Store, or invalid checksums.
: If you already have an earlier version of El Capitan (like 10.11.0) and just need to reach 10.11.6, use the OS X El Capitan 10.11.6 Combo Update Steps to "Fix" Common Installation Issues mac os x el capitan 10116 installer dmg download fixed
Most users encounter an error stating the installer "can't be verified" or "may have been tampered with." This is usually due to an expired security certificate. El Capitan full installer - Apple Communities If you are trying to download the OS
