Why won’t my Apple Watch pair to my phone?

Are you getting any error messages when you try to pair ?

Have you unpaired it from any previous iPhones before attempting to pair?

Make sure your iPhone is completely up to date on the latest software.

Try these steps:

1. Press and hold the Digital Crown and the side button at the same time until your watch restarts. ... 
2. After your watch restarts, firmly press the screen or press and hold the Digital Crown.
3. Tap Reset.
4. After your watch resets, you can pair it with your iPhone.

How to pair your Apple Watch with a new iPhone - Apple Support