You used a different Apple ID and password previously or you got this watch second hand from someone that previously owned it and don’t have their info to remove the lock preventing you from using it?
If you forgot your Apple ID password - Apple Support
If you go to settings on the watch (assuming it was previously yours and set up so that you can access settings) and click on your name (or whatever you used for your Apple ID profile) you should see the email address that was used as the Apple ID. You can then follow the steps in the link above to reset your Apple ID password. If this was not originally your watch you need to have the original owner do it or follow the steps to remove the watch from their Apple ID so you can use it. If you bought it and are not able to get the seller to do what is necessary you won’t be able to use the watch.