I am trying to post json invoice data i.e inv_id , Name etc & product data i.e item_id, item_name etc in database but getting error

my controller : public function store(Request $request) {

    $s = new Invoice();
    $s->inv_id = $request->inv_id;
    $s->inv_date_of_purchase = $request->inv_date_of_purchase;
    $s->inv_return_month = $request->inv_return_month;
    $s->inv_due_date = $request->inv_due_date;
    $s->inv_customer_name = $request->inv_customer_name;
    $s->inv_customer_gstin = $request->inv_customer_gstin;
    $s->inv_customer_state = $request->inv_customer_state;
    $s->inv_customer_bill_address = $request->inv_customer_bill_address;
    $s->save();

    $data =  $request->items;



    foreach($data->item_name as $key => $value) {
        InvoiceItem::create(['item_name'=>$value]);
    }

    return response()->json(['success'=> $data]);



}

blade post request:

$('#invsubmit').click( function(e) { e.preventDefault();

        var rows = [];
        $('#inv_table tbody tr').each(function(i, n){
            var $row = $(n);
            rows.push({
                item_id: $row.find('td:eq(0)').text(),
                item_name: $row.find('td:eq(1)').text(),
                item_type: $row.find('td:eq(2)').text(),
                item_hsn_sac: $row.find('td:eq(3)').text(),
                item_qty: $row.find('td:eq(4)').text(),
                item_unit: $row.find('td:eq(5)').text(),
                item_rate: $row.find('td:eq(6)').text(),
                item_tax_value: $row.find('td:eq(7)').text(),
                item_tax_rate_percentage: $row.find('td:eq(8)').text(),
                item_tax_cgst_percentage: $row.find('td:eq(9)').text(),
                item_tax_sgst_percentage: $row.find('td:eq(10)').text(),
                item_tax_igst_percentage: $row.find('td:eq(11)').text(),
                item_tax_cess_percentage: $row.find('td:eq(12)').text(),
                item_tax_cess: $row.find('td:eq(13)').text(),
                item_total: $row.find('td:eq(14)').text()
            });
        });


        var _token = $("input[name='_token']").val();
        var inv_id = $("input[name='inv_id']").val();
        var inv_date_of_purchase = $("input[name='inv_date_of_purchase']").val();
        var inv_return_month = $("input[name='inv_return_month']").val();
        var inv_due_date = $("input[name='inv_due_date']").val();
        var customername = $("input[name='customername']").val();
        var inv_customer_gstin = $("input[name='inv_customer_gstin']").val();
        var inv_customer_state = $("input[name='inv_customer_state']").val();
        var inv_customer_bill_address = $("textarea[name='inv_customer_bill_address']").val();
        var invdata = $('#inv_table td').text();







        var d = {

            _token:_token,
            inv_id:inv_id,
            invoice_id: inv_id,
            inv_date_of_purchase:inv_date_of_purchase,
            inv_return_month:inv_return_month,
            inv_due_date:inv_due_date,
            customername:customername,
            inv_customer_gstin:inv_customer_gstin,
            inv_customer_state:inv_customer_state,
            inv_customer_bill_address:inv_customer_bill_address,
            items:rows

        };
        console.log(JSON.stringify(d));

        $.ajax({
            url: "{{route('inv.store.post')}}",
            type:'POST',
            data: d

            /*{

                _token:_token, inv_id:inv_id,
                inv_date_of_purchase:inv_date_of_purchase,
                inv_return_month:inv_return_month,
                inv_due_date:inv_due_date,
                customername:customername,
                inv_customer_gstin:inv_customer_gstin,
                inv_customer_state:inv_customer_state,
                inv_customer_bill_address:inv_customer_bill_address,
                item:rows

            }*/


        });

       // alert(data);


    });

json data i am gettiing on alert(data)

{ "_token": "hjaWX7HS33eJ7pKyEsfX9xW6neCxNrOJU6CJ38aV", "inv_id": "", "invoice_id": "", "inv_date_of_purchase": "03/01/2018", "inv_return_month": "01/2018", "inv_due_date": "03/01/2018", "inv_customer_gstin": "21AAFCK9443B1ZK", "inv_customer_state": "West Bengal", "inv_customer_bill_address": "Sukantapally (East), Near shiv shakti kali mandir Siliguri Siliguri West Bengal India 734005", "items": [{ "item_id": "1", "item_name": "Amul Taza", "item_type": "", "item_hsn_sac": "3132518", "item_qty": "1", "item_unit": "cm", "item_rate": "24", "item_tax_value": "1.2", "item_tax_rate_percentage": "5", "item_tax_cgst_percentage": "2.5", "item_tax_sgst_percentage": "2.5", "item_tax_igst_percentage": "5", "item_tax_cess_percentage": "", "item_tax_cess": "", "item_total": "24" }, { "item_id": "2", "item_name": "Amul Taza", "item_type": "", "item_hsn_sac": "3132518", "item_qty": "1", "item_unit": "cm", "item_rate": "24", "item_tax_value": "1.2", "item_tax_rate_percentage": "5", "item_tax_cgst_percentage": "2.5", "item_tax_sgst_percentage": "2.5", "item_tax_igst_percentage": "5", "item_tax_cess_percentage": "", "item_tax_cess": "", "item_total": "24" }] }

my model: