将整个代码块封装在一个异步函数中，并在函数中使用await来等待Ticket.find操作的结果。

exports.OrderPlace = async (req, res) => {
  const all_product = req.body.product_info;
  let meta = [];
  let flag = "";

  for (const element of all_product) {
    const docs = await Ticket.find({ _id: element.product_id }).exec();
    if (docs[0]._id == element.product_id) {
      if (element.quantity > docs[0].ticket_quantity) {
        flag = "false";
        meta.push({
          cart_id: element.id,
          pro_id: element.product_id,
          quantity: element.quantity + " Asking quentity is not available!",
        });
      }
    }
  }
  console.log({ flag: flag, meta });
};

我发现操作对象数组最简单的方法是使用数组方法，如filter、map等...
下面的代码可能会帮助您

exports.OrderPlace = (req, res) => {
  const all_product = req.body.product_info;
  let meta = [];
  all_product.forEach((element) => {
    Ticket.find({ _id: element.product_id }, function (err, docs) {
      if (err) return handleError(err);

      element = element.filter((item) => {
          return (docs[0]._id == element.product_id && element.quantity < docs[0].quantity)
      })

      meta = element.map((item) => {
            return {
                cart_id: element.id,
                pro_id: element.product_id,
                quantity: element.quantity + " Asking quentity is not available!",
            }
      });
    });
  });
  console.log(meta);
};