The safest and most reliable way to download macOS Sierra is through the official Apple Support website. Unlike newer versions, Sierra does not always appear in the Mac App Store search results.
. While modern versions of macOS have moved toward the Sonoma and Sequoia eras, Sierra remains a vital "bridge" operating system for specific professional and technical workflows. The Purpose of Sierra Today
Released in July 2017, version 10.12.6 was the final stable update for the macOS Sierra lifecycle. It focused heavily on stability, security, and enterprise compatibility, addressing issues with SMB connections and the Terminal app. It was the last major release to use the HFS+ file system as the primary standard before Apple transitioned to APFS with High Sierra. System Compatibility
Apple provides specific links for older macOS versions. Since this is an older OS, it may not appear in the standard App Store search.