Hello anurag_vishnoi,
Congratulations on your new iPhone. We understand that you're having issues pairing your Apple Watch with your new iPhone, and we'd like to help.
Which Apple Watch model are you trying to pair with your new iPhone 11? In order to be able to pair your Apple Watch with your new iPhone, it has to be Series 2 or later.
If so, please restart your both your iPhone and Apple Watch:
Restart your iPhone, iPad, or iPod touch
How to restart your Apple Watch
After restarting both devices, please try to update, and pair them again at least three times.
If that doesn't work, unplug your Wi-Fi router, wait 30 seconds then plug it back in, and try to update, and pair again.
If the issue persist, the next step we would like to try is, resetting your network settings. You can do this by going to: Settings > General > Reset > Reset Network Settings. Keep in mind, this also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.
After completing this these steps, attempt updating, and pairing.
If that didn't help, try connecting to a different Wi-Fi network.
Hope this helps. Kind regards.