activation lock

Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.

About Activation Lock on your Apple Watch - Apple Support

Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. Try to get a refund (through eBay) or dispose of the watch properly. In the future, see the 'Before you buy a watch from a third-party' section in:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Here is how the previous owner can unpair the watch if he no longer has the paired iPhone and remembers his Apple ID & password:

• With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

ender_20 wrote:

Bough a used apple watch and the previous owner doesn't have the old log in to their old icloud account as they lost their previous phone which had their old icloud account.

Return it for a refund. That’s your best option here, if the seller is still available.

If the seller still has the original purchase receipt should the serial number (or IMEI, if a cellular model) and can provide that receipt to you, that might be enough to convince Apple to unlock it for you.

If the seller has disappeared as is often the case, or if the sale was “as is”, then recycle the Watch as it will never be useful to you. We can’t help. Apple won’t. Yes, recycle it, really.

The following is written for iPad and iPhone, but also applies to Watch:

… The All Too Common SAD Reality of Buying … - Apple Community