Error: Sync feature not available

gistfile1.txt

var express = require('express');
var app = express();
const Realm = require('realm');

// realm set up
const adminUser = Realm.Sync.User.adminUser('**');
var token = "***";
Realm.Sync.setAccessToken(token);

const AppointmentSchema = {
    name: 'Appointment',
    primaryKey: 'id',
    properties: {
        id: 'string',
        phoneNumber: 'string',
        message: 'string',
        time: 'date',
        reminderTime: {type: 'date', indexed: true}
    }
};

const appointmentRealm = new Realm({ 
            sync:  { // error still happening here
                user: adminUser, 
                url: "realm://***/appointments"
            }, 
            schema: [AppointmentSchema] 
        });