Cannot reinstall MacOS

Helllo,

I own a Macbook Pro from 2017, which I'm attempting to factory reset. As instructed from tutorial, Ive eased the using the disk unity application. When I go to reinstall MacOS (Sierra) at the very end I keep getting the message 'an error occurred while preparing the installation. Try running this application again. I cannot seem to get pass this stage. Ive gone to repair (first aid) the disk, but agin this has not fixed the problem. Any advice would be welcomed.