Before downloading the installer, ensure that your Mac is compatible with macOS 10.12. Attempting to install Sierra on an unsupported machine can lead to software instability or installation failures. Supported Mac Models Late 2009 or newer MacBook Air: Late 2010 or newer MacBook Pro: Mid 2010 or newer Mac mini: Mid 2010 or newer iMac: Late 2009 or newer Mac Pro: Mid 2010 or newer Minimum Hardware Specifications

Officially, Apple hides old OS installers. If you search the App Store today, you will likely be redirected to macOS Ventura or Sonoma, which your old Mac cannot run.

Apple provides a built-in tool called createinstallmedia hidden inside every macOS installer. Assuming you have moved the Install macOS Sierra.app to your Applications folder:

"This copy of the Install macOS Sierra application is damaged"

"This copy of the Install macOS Sierra application damages can't be used" Error

This error occurs because Apple’s security certificates for older installers have expired.

Whether you are reviving a 2009 MacBook Pro, running legacy mixing software, or simply exploring macOS history, Sierra remains a rock-solid operating system. Now that you have the full installer, back it up to an external drive and an archive service—because Apple may finally pull Sierra from its servers in the coming year.

Once you have successfully downloaded the full installer into your Applications folder, creating a bootable USB drive is the best way to perform a clean installation. Prerequisites A USB flash drive with at least of capacity.

curl -O https://swcdn.apple.com/content/downloads/00/42/061-28683/owvxi67czq832leirn2heal5ovhbhymd7y/InstallMacOS.pkg

Download macOS Sierra 10.12 InstallOS.dmg