For a moment, nothing happened. The cooling fans of the 2015 MacBook Pro whirred, a sound like a jet engine preparing for takeoff. Elias was a data recovery specialist, a cleaner for people who had deleted things they shouldn't have, or in this case, a restorer of systems that had been corrupted by the relentless march of progress.

Since Apple doesn't directly provide links for older macOS versions, and to avoid any potential risks, I recommend against downloading from third-party sites. However, if you have access to a friend's Mac that already has Sierra installed or a Time Machine backup of a Mac with Sierra, you can use that.

This guide aims to help you through the process while emphasizing the importance of adhering to software licensing agreements and focusing on safe and recommended practices.

Apple still hosts Sierra on their official support site.

Before proceeding with any major update or installation, it's crucial to back up your Mac. Use Time Machine or any other backup method you prefer to ensure your data is safe.

You can create a bootable installer for macOS Sierra on a USB drive using another Mac with the Sierra installer downloaded. You can use tools like DiskMaker X or Terminal commands to create a bootable installer.