Activation Lock on your Apple Watch

Contact the person you bought it off and get them to follow these steps;

What to do before you sell or give away your old watch

If they can't or won't then get your money back as the watch will be of no use to you.

It was not prepared correctly, please return it or contact the seller to fix the problem. Have them refer to https://support.apple.com/en-us/HT208855

Apple watches don’t have iCloud, what are you referring to?