Apple watch

You may have to update the phone iOS.

To update an iPhone 8 or later go to Settings>General>Software Update and scroll to the bottom of the screen.

What model of phone is it and what version of ios is it running?

What model of watch is it and what version of WatchOS is it running?.

What troubleshooting steps have you tried?.

What errror messages are you receiving?.

Has this watch ever been paired to another phone?(Very important).