Hi
Your Apple Watch has previously been updated with watchOS 4 developer beta software.
It therefore cannot be paired to an iPhone running iOS 10.x
watchOS is not user-downgradable.
Your options include:
- Returning the Apple Watch to the seller for a refund.
- Waiting for iOS 11 to be released (due for release this coming autumn / fall).
- Participating in the iOS 11 public beta (this is not recommended for your main device):
- Contacting Apple Support or making a Genius Bar reservation to establish your available service options and any cost for returning the Apple Watch to watchOS 3.x..