Ajax call to web service giving me message=invalid web service call, missing value for parameter: 'json'." error

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ASP.NET

Ajax

web

i am getting error.i am not able to pass drop down values.can any one help me here is my functions are

ajax:

$(document).ready(function () {

    $("#ddlDistrict").change(function () {

        var selectedItem = $("#ddlDistrict").val();
        var ddlTaluk = $("#ddlTaluk");
        $("#ddlVillage").empty();
        Getalltaluksbydistrict(selectedItem);

    });

    $("#ddlTaluk").change(function () {
        var selectedItem = $("#ddlDistrict").val();
        var selectedItem1 = $("#ddlTaluk").val();
        var ddlDistrict = $("#ddlDistrict");
        var ddlTaluk = $("#ddlTaluk");
        GetVillagedetailsbyTaluk(selectedItem, selectedItem1);

    });

    //Getalldistricts();
    Getalldistricts();

    function Getalldistricts() {
        $.ajax({
            type: "POST",
            url: "School.asmx/Getalldistricts",
            data: "{}",
            contentType: "application/json;character=utf-8",
            dataType: "json",
            success: OnSuccess,
            error: OnError
        });

        function OnSuccess(data, status) {
            var obj = JSON.parse(data.d);
            $("#ddlDistrict").empty();
            $("#ddlDistrict").append('<option value="">--Select District--</option>');
            for (var i = 0; i < obj.length; i++) {
                $("#ddlDistrict").append('<option value="' + obj[i].districtna + '">' + obj[i].districtna + '</option>');
            }
        }
        function OnError(request, status, error) {
            alert(request.statusText);
        }
    }

    //Getalltaluksbydistrict(DistrictName)

    function Getalltaluksbydistrict(districtna) {
        $.ajax({
            type: "POST",
            url: "School.asmx/Getalltaluksbydistrict",
            data: "{'districtna':'" + districtna + "'}",
            contentType: "application/json;character=utf-8",
            dataType: "json",
            success: OnSuccess,
            error: OnError

        });

        function OnSuccess(data, status) {
            var obj = JSON.parse(data.d);
            $("#ddlTaluk").empty();
            $("#ddlTaluk").append('<option value="">--Select Taluk--</option>');
            for (var i = 0; i < obj.length; i++) {
                $("#ddlTaluk").append('<option value="' + obj[i].talukname + '">' + obj[i].talukname + '</option>');
                //alert(obj[taluknamekan]);
            }
        }
        function OnError(request, status, error) {
            alert(request.statusText);
        }
    }

    function GetVillagedetailsbyTaluk(districtna, talukname) {
        $.ajax({
            type: "POST",
            url: "School.asmx/GetVillagedetailsbyTaluk",
            data: "{'districtna':'" + districtna + "', "
                + "'talukname':'" + talukname + "' }",
            contentType: "application/json;character=utf-8",
            dataType: "json",
            success: OnSuccess,
            error: OnError

        });

        function OnSuccess(data, status) {
            var obj = JSON.parse(data.d);
            $("#ddlVillage").empty();
            $("#ddlVillage").append('<option value="">--Select Village--</option>');
            for (var i = 0; i < obj.length; i++) {
                $("#ddlVillage").append('<option value="' + obj[i].villagenam + '">' + obj[i].villagenam + '</option>');
                //alert(obj[taluknamekan]);
            }
        }
        function OnError(request, status, error) {
            alert(request.statusText);
        }
    }

    $('#btnsubmit').click(function () {
       
        var ddldistrictvalue =$("#ddlDistrict").val();
        var ddltalukvalue=$("#ddlTaluk").val();
        var ddlvillagevalue=$("#ddlVillage").val();
        var txtschcode=$("#txtschcode").val(); 
        var txtschname =$ ("#txtschname").val();
        var txtdistclu = $("#txtdistclu").val();
        var txtlowvalue = $("#txtlowvalue").val();
        var txtupvalue = $("#txtupvalue").val();
        var txtprpsec = $("#txtprpsec").val();
        var txtprpstud = $("#txtprpstud").val();
        var prpteach = $("#prpteach").val();
        var txtworkday = $("#txtworkday").val();
        var txtshftsch = $("#txtshftsch").val();
        var txtacainsp = $("#txtacainsp").val();
        var txtcrc = $("#txtcrc").val();
        var txtbrc = $("#txtbrc").val();
        var txtappro = $("#txtappro").val();
        var txtestyr = $("#txtestyr").val();
        var txtyor=$("#txtyor").val();
        var yorpri=$("#yorpri").val();
        var ddlloct = $("#ddllocat").val();

        insertdetails(ddldistrictvalue, ddltalukvalue, ddlvillagevalue, txtschcode, txtschname, txtdistclu, txtlowvalue,
           txtupvalue, txtprpsec, txtprpstud, prpteach, txtworkday, txtshftsch, txtacainsp, txtcrc, txtbrc, txtappro, txtestyr,
           txtyor, yorpri, ddlloct);
    });

    function insertdetails(ddldistrictvalue, ddltalukvalue, ddlvillagevalue, txtschcode, txtschname, txtdistclu, txtlowvalue,
           txtupvalue, txtprpsec, txtprpstud, prpteach, txtworkday, txtshftsch, txtacainsp, txtcrc, txtbrc, txtappro, txtestyr,
           txtyor, yorpri, ddlloct) {
      

        $.ajax({
            type: 'POST',
            contentType: "application/json; charset=utf-8",
            url: "School.asmx/insertschooldetails",
            data: "{'districtna':'" + ddldistrictvalue + "','talukname':'" + ddltalukvalue + "','villagenam':'" + ddlvillagevalue + "',"
                + "'schcd':'" + txtschcode + "','districtna':'" + txtschname + "','distcrc':'" + txtdistclu + "',"
                + "'lowclass':'" + txtlowvalue + "','highclass':'" + txtupvalue + "','ppsrc_yn':'" + txtprpsec + "',"
                + "'ppstudent':'" + txtprpstud + "','ppteacher':'" + prpteach + "','workdays':'" + txtworkday + "',"
                 + "'schshi_yn':'" + txtshftsch + "','noinspect':'" + txtacainsp + "','visitscrc':'" + txtcrc + "',"
                 + "'visitsbrc':'" + txtbrc + "','approachby':'" + txtappro + "','estdyear':'" + txtestyr + "',"
                 + "'yearrecog':'" + txtyor + "','yearupgrd':'" + yorpri + "','rururb':'" + ddlloct + "'}",
            async: false,
            success: function (response) {

                alert("Record saved successfully..!!");
            },
            error: function () {
                alert("Error");
            }
        });
    }
 

});

webservices:

[WebMethod]
      public String insertschooldetails(String ddlDistrict, String ddlTaluk, String ddlVillage,
               String txtschcode, String txtschname, String txtdistclu, Int32 txtlowcl, Int32 txtupcl, String txtprpsec, String txtprpstud, String prpteach, String txtworkday, String txtshftsch, String txtacainsp, String txtcrc, String txtbrc,
               String txtappro, String txtestyre, String txtyor, String yorpri, String ddllocat)
      {
          DataTable dt = SchoolDetailsBAL.insertschooldetails( ddlDistrict, ddlTaluk, ddlVillage,
                txtschcode, txtschname, txtdistclu, txtlowcl, txtupcl, txtprpsec, txtprpstud, prpteach, txtworkday, txtshftsch, txtacainsp, txtcrc, txtbrc,
                txtappro, txtestyre, txtyor, yorpri,  ddllocat);
          return JsonConvert.SerializeObject(dt);
      }

What I have tried:

i have tried with JSON.stringify .still i am getting same error