I have an insurance for my Apple Watch, but I don’t have an Apple Watch anymore. How can I cancel the payment?

Follow the instructions here, Cancel or change the payment method for your AppleCare plan - Apple Support

Are you sure you are referring to insurance, or are you referring to AppleCare? Apple does not have insurance on their devices, that would have to come from a 3rd party. AppleCare or AppleCare+ are extensions of the warranty for your Apple product. If you explain what you are referring to, someone should be able to tell you what to do.