Before trying again to pair your watch, it may help to close all open apps on your iPhone and then to restart both devices:
- On your iPhone, double-click on the Home button, then swipe up on each app preview to close it;
- Turn both devices off together, then restart your iPhone first;
- To turn your watch off: press and hold the side button until you see the Power Off slider; drag it to turn off.
- To turn it back on: press and hold the side button until you see the Apple logo.
You may then also be able to follow the standard pairing process, without needing to pair manually:
Set up your Apple Watch - Apple Support