The macOS Sierra 10.12.6 Combo Update is and can only be installed on a Mac that already runs macOS Sierra (any version from 10.12.0 to 10.12.5). Many users mistakenly download this update, attempt to run it on El Capitan, and receive an error message indicating that Sierra must be installed first. This update cannot function as a standalone operating system installer.

Retrieve the official deployment installer package by navigating to the legacy repository hosted at Apple Support: Download and Install macOS .

Clicking this link downloads a file named InstallmacOSX.dmg directly from Apple's servers.

– Use this if you are already running an earlier version of Sierra (10.12.0 - 10.12.5).