I bought a Apple Watch offline from a group on Facebook but I’m unable to log onto it and can’t find the person on Facebook to ask about it.
Thanks for reaching out to the Apple Support Communities! We understand you have some questions regarding an Apple Watch that was purchased used. We are happy to help provide some guidance.
Before an Apple Watch is given away or sold, the previous owner would need to sign out of their Apple ID account to disable the Activation Lock feature. The previous owner would need to do this prior to selling the device. You can find the steps on how this is done in the following article in the section titled "Turn off Activation Lock before you send your Apple Watch in for service, sell it, or give it away":
About Activation Lock on your Apple Watch
You can find additional information on removing activation lock here:
How to remove Activation Lock