Each seller of the Watch should have done these things before passing it onto the next owner. That includes removing the Find My setting which is what is giving you problems.
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
This support article has information about Activation Lock on your Apple Watch and how to turn it off. --> About Activation Lock on your Apple Watch - Apple Support
If you cannot remove the lock using the account information and you still happen to have the original receipt, you can apply to Apple to have them reset it.
- Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by an Apple Authorized dealer and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership). Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support