How to pair Apple Watch if purchased from someone?

Make sure that the watch that you're about to buy from someone else is ready for you to use. --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

Learn how to set up your new Apple Watch with your iPhone by reading the information in this support article -->

Set up your Apple Watch - Apple Support

"To set up and use your Apple Watch, you need an iPhone with the latest version of iOS."

"When you set up Apple Watch, your iPhone asks you to hold the watch up to your iPhone camera. If your iPhone says that it's unable to connect to Apple Watch — or the watch shows a clock face — make sure that Apple Watch isn't already paired." If you can't pair an Apple Watch with your iPhone see if the suggestions in this document help -->

If you can't pair an Apple Watch with your iPhone - Apple Support

I'm going to guess the watch is activation locked and wasn't prepared properly for a new user? If that is your reality, you can't use the watch until the "person" you bought the watch from removes the watch from their account.

Can you reach the person you bought the watch from? If yes, send them this link (it's not something you can do yourself) --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

If you can't reach whoever you purchased the watch from, you've likely got a useless watch you cannot use. Apple won't help you and we cannot help you. If you can't get your money back, recycle the watch responsibly and learn from this experience.

Read this as well --> The All Too Common SAD Reality of Buying … - Apple Community