Apple Watch is still on previous owners account

More specifically the seller needs to release the watch's Activation Lock to their Apple ID. This can be done remotely online.

Remove  Watch Activation Lock -

Go to icloud.com and login

Open Find iPhone.

In the All Devices pulldown menu select your  Watch.

In the resulting sheet that appears to the right choose erase Apple Watch. Select Next until the device is erased.

Then Remove the watch from your Apple ID.

This should reset the watch to the factory settings for you to setup as new and pair it to your iPhone.

If the seller won't do this or can't do this, you've been scammed.

M1 Mac mini, macOS 13.1; Watch, series 7 GPS/cellular, watchOS 9.2; iPad Pro 2nd Gen, iPadOS 16.2; iPhone 12 Pro, iOS 16.2; TV 4th Gen, tvOS 16.2; HomePods, audioOS 16.2 and Airport Express w/ Airplay 2

The previous owner will need to remove it from their icloud