EDIT:
Apple’s readded sierra to MAS
https://itunes.apple.com/us/app/macos-sierra/id1127487414?mt=12
One of the changes that came with High Sierra’s release and affected fellow macadmins was the removal of Sierra from the app store.
For now, there’s a work around as Apple still has Sierra in the Software Update Catalogue
- Sign into the Mac App Store with an account that had Sierra
- Download Mas CLI https://github.com/mas-cli/mas
- Extract mac cli
- Drag mas into the Applications folder
- Open Terminal located in Applications -> Utilities
- type
cd /Applications/
- type
./mas install 1127487414This will download the installer Application
- Wait