i have always ole watch app on phone. iOS 11
click on pair
hold up to camera
comes back
can’t pair watch to iPhone
Another possibility is that the prior owner updated the Series 2 to watchOS 5, which requires iOS 12 on your iPhone.
Has the previous owner removed the watch from his/her iCloud account and removed Activation Lock?
Do you see any other error on the Phone or the Watch with this? Try this for troubleshooting, If your Apple Watch isn't connected or paired with your iPhone - Apple Support
Can’t pair a used Apple Watch series 2
