SQL ERROR

I’m trying to insert datas to my purchase_order table and purchase_order_item simultaneously, my problem is either, (intermediate value) is not iterable or getting the sql format error

Please give me tips and advice, I’m new to coding
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  const { supplier_Id, order_date, items } = req.body;

  if (
    !supplier_Id ||
    !order_date ||
    !items ||
    !Array.isArray(items) ||
    items.length === 0
  ) {
    return res.status(400).json({
      message: "All fields are required and items must be a non-empty array.",
    });
  }

  const conn = await pool.getConnection();

  try {
    await conn.beginTransaction();

    console.log("Connection established");

    const purchaseOrderResult = await conn.query(
      "INSERT INTO purchase_order (supplier_Id, order_date) VALUES (?, ?)",
      [supplier_Id, order_date]
    );

    const purchaseOrderId = purchaseOrderResult.insertId || [];
    console.log("Purchase Order Result:", purchaseOrderId);

    const purchaseOrderItems = [];
    for (const item of items) {
      const [productResult] = await conn.query(
        "SELECT pieces_per_box FROM products WHERE productId = ?",
        [item.productId]
      );

      if (productResult.length === 0) {
        throw new Error(`Product with ID ${item.productId} not found`);
      }

      const piecesPerBox = productResult.pieces_per_box || [];
      const totalQuantity = item.quantity * piecesPerBox;

      purchaseOrderItems.push([purchaseOrderId, item.productId, totalQuantity]);
    }

    if (purchaseOrderItems.length > 0) {
      await conn.query(
        "INSERT INTO purchase_order_item (fk_purchaseOrder, fk_productId, quantity) VALUES ?",
        [purchaseOrderItems]
      );
    }
    console.log(purchaseOrderItems);

    await conn.commit();

    conn.release();
    res.status(201).send("Purchase Order added successfully");
  } catch (error) {
    await conn.rollback();
    console.error("Error adding Purchase Order:", error);
    res.status(500).json({ error: "Error adding Purchase Order" });
  }
});```