|Reasons aside, to answer your question you've typed the command wrong (wrong syntax). It should be:|
format c: /s
But if your goal is to re-install a fresh Windows, all you need to do is boot the PC from the Windows installation media and it will allow you format partitions, delete them, or create them, all without typing commands. You just choose from the options on-screen.
There's been no need to manually enter the 'format' command for years.
Even if you boot from a partition management utility disc, formatting involves nothing more than a couple of mouse clicks.
You're using "old school" methods, nothing wrong with doing that if you cut your teeth on DOS & understand the commands - - but you don't.
message edited by phil22