Hi
It may help to follow the steps here under "Get help":
Set up your Apple Watch - Apple Support
If your Apple Watch has previously been running watchOS 4 developer beta, then watchOS cannot be downgraded by the user and the Apple Watch cannot be paired to an iPhone running iOS 10.x. Your options are:
- Wait for the public version of iOS 11 to be released, then update your iPhone and re-pair your Apple Watch.
- Install iOS 11 beta on your iPhone and then pair your Apple Watch:
- Note that Apple advises against using beta software on your primary devices, as the software may contain errors or inaccuracies and may not function as well as commercially released software.
- Apple Beta Software Program (FAQs)
- Apple Beta Software Program
- Contact Apple Support or make a Genius Bar reservation to establish your options for downgrading the software on your Apple Watch.