Can you try enclosing your object in ``. That is,
tracking_info = `{ "fulfillment": { "location_id": '55957913762', "tracking_company": "TNT", "tracking_number": "123456789", "tracking_url": "www.baidu.com", 'order_id' : '4945712251042', "line_items":[ { 'id': '11718958743714' } ], "notify_customer": False } }`;
shopify.Fulfillment.create(tracking_info)