i sold my apple watch 3 i thought i unlocked it but the person who bought it said its still locked. i cant seem to find it anywhere

I recently sold my Apple Watch three I thought I unlocked it when the person bought it off eBay he sent me a picture showing my email was still locked to the watch I checked in Settings and I cannot seem to find my Apple Watch three I see my watch and I’m currently using how do I unlock my Apple Watch three so this person can use it