Return WP_Error as WP_REST_Response

I have a http request to WP REST API that can resolve in a success or a failure status

The success gets handled by WP_REST_Response, and if any error occurs this is handled by WP_Error, both returned if that's the case

However, in both cases when I receive the server response I don't get any of the messages I've added

    public function myFunction($request = null)
    {
        $response = array();
        $parameters = $request-get_json_params();
        $error = new WP_Error();

        if (success) {
                $response['code'] = 200;
                $response['message'] = __("Great", "great");
            } else {
                $error-add(406, __("nope", 'almost'), array('status' = 400));
                return $error;
            }

        return new WP_REST_Response($response, 200);
    }

If I inspect the response that I receive, I cannot have access to any message, I only receive

Response {
  body: (...)
  bodyUsed: true
  headers: Headers {}
  ok: true
  redirected: false
  status: 200
  statusText: "OK"
  url: "..."
  __proto__: Response
}

How does it work?