How do I setup my Apple Watch on a new iPhone?

Remove watch from your old phone;

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.

Then;

Setup your Watch to a new iPhone

If you already tried (to no avail) all the steps provided in the support article that led you here, contact Apple Support directly using this link:--> Get Support, or make an appointment (*) and seek for assistance at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking your devices, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.