Pairing problem Series 3 Apple Watch to iPhone X

If the message you are receiving says unpairing failed it means the person who sold it to you did not unpair it from their iPhone. You probably will run into Activation Lock next. Contact the seller and send them this: What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

See if this helps....https://support.apple.com/en-us/HT205025

If not, click on the "Contact Support" link at the upper right side of this page.

Then reset it by pressing in on the crown and side button until you see the Apple logo, then release both buttons.

If that doesn't work make an appointment with an Apple store Genius Bar and bring your Watch and iPhone with you.