Activation lock for sold apple watch

So, i sold the apple watch and the owner contacted me saying it has activation lock.

Here's a list of things I've done.

-I've gone into my I cloud settings & the device isn't linked to my account.

-The new owner has tried resetting/restarting the apple watch.

-Tried to contact support but they require serial ID but the owner cant get that if he cant get into the apple watch.

-The apple watch is not connected to the apple watch app.

If anyone has any ideas that would be appreciated cheers.