Have you run Disk First Aid in Disk Utility yet?
Which method are you using?
A bootable USB stick?
Sierra won’t install to SSD, thanks to Eau Rouge…
OS Sierra on an external drive - Apple Community
Or...
To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… How to reinstall macOS - Apple Support
Command (⌘)-R
Install the latest macOS that was installed on your Mac.
Option-⌘-R
Upgrade to the latest macOS compatible with your Mac.
Shift-Option-⌘-R
Install the macOS that came with your Mac, or the closest version still available.