How to reset my Apple Watch Series 6
Can’t paired to my iPhone 10
Question marked as
Which watchOS is installed on your Apple Watch 6?
If watchOS 11, you will need at least an iPhone XS or later with iOS 18 installed.
In case watchOS 10 is installed, your iPhone needs to have iOS 17.
Have a look at the related articles:
Set up your Apple Watch - Apple Support
If you can't pair an Apple Watch with your iPhone - Apple Support