Download macOS Sierra Final from Mac App Store – Direct Link : Download macOS Sierra 10.12 Final
App store, sign in using your apple ID. (if you've already purchased )Mac OS Sierra.
Extract it to a folder, you'll get an executable file : "mas"
Drag "mas" file to terminal window, then behind it type so it says like this:
<yourusername>$ /Users/@@@/Downloads/mas-cli/mas install 1127487414
Will start downloading latest Sierra (10.12.6)
just like using app store.