I want to sell my apple watch, I am being asked to unpair it from my phone, which was sold months ago. How do I proceed?

To unpair and erase watch and remove Activation Lock

From a web browser,sign in to iCloud.com with your Apple ID.

Go to Find iPhone.

Click All Devices at the top of the screen.

Click your Apple Watch.

Click Erase [device]. Select Next until the device is erased.

Click Remove from Account.

You don't need the phone,that's why I posted that solution,your alternative is;