How can I remove the Activation Lock on my Apple Watch Series 6?

Sorry, but only the previous owner can disable the Activation Lock for the Apple Watch, which can also be done via the iCloud Account over the Web:

About Activation Lock on your Apple Watch - Apple Support

Unless you have proof of purchase or ownership, as explained in this related article, the official Apple Support can not disable the Activation Lock:

Additional Support - Activation Lock

Good afternoon, The activation lock can only be by the owner of the device, if you are the owner, here is what you do- On your iPhone-Go to settings>General>VPN> and click device management, pick the device that is locked and then you should have the option to clear it.

Extra if my instructions were not clear- Activation Lock settings with Profile Manager - Apple Support