The only legit location for a download of any macOS version is from Apple. If you try it from any other source, you're on your own. Pirated versions may be infested with malware. Yes, if not from Apple, it's pirate ware.
If you google how to download Sierra OS, you should find a link from Apple, that will rout you to the App Store to download Sierra. It’s tricky you can’t just go to the App Store and search it