I sold my Apple Watch but it doesn’t want to erase off my iCloud and let user use

Try this:

• With a web browser, go to: https://www.iCloud.com and sign in with your Apple ID. Open Find My iPhone and select All Devices (at the top). Then click your Apple Watch, select 'Erase Apple Watch', and select Next until the device is erased. Finally click "X" next to remove your Apple Watch.

And see:

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support