I called Apple Support and was told that you cannot install MacOS Sierra from the MacOSSierra app that you download from the Apple Store. It can only be installed as an upgrade from a current OS. I asked why and he said it's because of software piracy.
That sounds like nonsense!
I f I d/l it and then give it to you while it's still running on my deuced Mac(hina), that's technically piracy, but otherwise...?
I d/l'ed Sierra and installed it as a new installation, NOT an upgrade.
Not everybody even wants to upgrade as opposed to doing a clean install.
More: The d/l should be "Install macOS Sierra.app". (I'll email it to you if you'd like, and it won't be piracy, because I never did an installation with this particular d/l.)