Hi
The following steps may help:
If you have not yet done so, remove the Apple Watch from Find My iPhone online:
- On a computer, in a web browser, go to: https://www.iCloud.com > sign in with your Apple ID.
- Open Find iPhone > select All Devices (at the top) > click your Apple Watch.
- Click Erase Apple Watch > Select Next until the device is erased.
- Click "X" next to your Apple Watch.
If you have not yet done so and if the option remains available, also unpair your watch from within the Watch app on your iPhone:
- On your iPhone, in the Watch app, go to: My Watch (tab) > tap on the name of the watch (at the top) > tap on the "i" icon next to the watch > if the button is shown, tap on Unpair Apple Watch and follow the further instructions.
Having followed the previous steps, ask the new owner to follow these:
On their iPhone: close the Watch app (double-press on the Home button, then swipe up on the Watch app preview).
Restart both their iPhone and watch, turning both off together, then restarting their iPhone first:
Re-try pairing and setting up their watch:
If the watch remains locked, either:
Accompany the buyer and the watch to a Genius Bar appointment, taking along your original proof of purchase for the watch.
Or contact Apple Support (you, not the new owner) and explain that you have been unable to remove Activation Lock from a watch that you have sold / given away to a new owner:
More information:
About Activation Lock on your Apple Watch - Apple Support