MacOS is not copy-protected in the usual sense:
Apple software downloaded from the Mac App Store, including MacOS, is not copy protected. However, it IS tagged with the Apple-ID of the purchaser when "purchased" (or equivalent — Think of MacOS as purchased on sale for $0.) A valid Apple-ID is required for almost all downloads from the Mac App Store.
You cannot update, upgrade, or install new software based on already-installed software UNLESS your Mac supplies the Apple-ID of the original software.
If your old MacOS is damaged, or you completely erased your Mac, the Apple-ID is not available, and you get the “cannot contact the server…” message that should go on to say, “…with the Apple-ID provided or NO Apple-ID”.