After using the code below, I encountered an issue with storing the value of the second dropdownlist in the database when clicking on the update button. An error occurred referring to a null value. In the form.aspx.cs file, I attempted to retrieve the value by using

cmd.Parameters.AddWithValue("@department", DropDownList2.SelectedItem.ToString());

, but it ended up taking a null value.

 <script type="text/javascript">
    $(function() {
    //$('#<%=DropDownList2.ClientID %>').attr('disabled', 'disabled');
    $('#<%=DropDownList2.ClientID %>').append('<option selected="selected" value="0">Select Post</option>');
    $('#<%=DropDownList1.ClientID %>').change(function() {
    var country = $('#<%=DropDownList1.ClientID%>').val()
    $('#<%=DropDownList2.ClientID %>').removeAttr("disabled");
    $.ajax({
    type: "POST",
    url: "http://localhost:50384/update_add_staff.aspx/BindPosts",
    data: "{'country':'" + country + "'}",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function(msg) {
    var j = jQuery.parseJSON(msg.d);
    var options;
    for (var i = 0; i < j.length; i++) {
    options += '<option value="' + j[i].optionValue + '">' + j[i].optionDisplay + '</option>'
    }
    $('#<%=DropDownList2.ClientID %>').html(options)
    },
    error: function(data) {
    alert('Something Went Wrong')
    }
    });
    });
    $('#<%=DropDownList2.ClientID %>').append('<option selected="selected" value="0">Select Post</option>');
    $("#DropDownList2").children("option").is("selected").text() 
    $('#<%=DropDownList2.ClientID %>').change(function() {
    var stateid = $('#<%=DropDownList2.ClientID%>').val()
      var myVar =('#DropDownList2').find(':selected').text();
    $.ajax({
    type: "POST",
    url: "http://localhost:50384/update_add_staff.aspx/BindPosts",
    data: "{'state':'" + post_id + "'}",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function(msg) {
    var j = jQuery.parseJSON(msg.d);
    var options;
    for (var i = 0; i < j.length; i++) {
    options += '<option value="' + j[i].optionValue + '">' + j[i].optionDisplay + '</option>'
    }
    $('#<%=DropDownList2.ClientID %>').html(options)
    $("option:selected", myVar).text()
    },
    error: function(data) {
    alert('Something Went Wrong')
    }
    });
    })
    })
    </script>

In regards to my C# code:

 private void Bindcategorydown()
        {
            String strQuery = "SELECT [dept_no],[department_name] FROM [first].[dbo].[dept_add]";
            using (SqlConnection con = new SqlConnection(strcon))
            {
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.CommandText = strQuery;
                    cmd.Connection = con;
                    con.Open();
                    DropDownList1.DataSource = cmd.ExecuteReader();
                    DropDownList1.DataTextField = "department_name";
                    DropDownList1.DataValueField = "dept_no";
                    DropDownList1.DataBind();
                    DropDownList1.Items.Insert(0, new ListItem("Select Department", "0"));
                    con.Close();
                }
            }
        }
        [WebMethod]
        public static string BindPosts(string country)
        {
            StringWriter builder = new StringWriter();
            String strQuery = "SELECT [post_id],[post_name] FROM [first].[dbo].[add_post] where group_id=@dept_no";
            DataSet ds = new DataSet();
            using (SqlConnection con = new SqlConnection(strcon))
            {
                using (SqlCommand cmd = new SqlCommand())
                {
                    cmd.CommandType = CommandType.Text;
                    cmd.CommandText = strQuery;
                    cmd.Parameters.AddWithValue("@dept_no", country);
                    cmd.Connection = con;
                    con.Open();
                    SqlDataAdapter da = new SqlDataAdapter(cmd);
                    da.Fill(ds);
                    con.Close();
                }
            }
            DataTable dt = ds.Tables[0];
            builder.WriteLine("[");
            if (dt.Rows.Count > 0)
            {
                builder.WriteLine("{\"optionDisplay\":\"Select Post\",");
                builder.WriteLine("\"optionValue\":\"0\"},");
                for (int i = 0; i <= dt.Rows.Count - 1; i++)
                {
                    builder.WriteLine("{\"optionDisplay\":\"" + dt.Rows[i]["post_name"] + "\",");
                    builder.WriteLine("\"optionValue\":\"" + dt.Rows[i]["post_id"] + "\"},");
                }
            }
            else
            {
                builder.WriteLine("{\"optionDisplay\":\"Select Post\",");
                builder.WriteLine("\"optionValue\":\"0\"},");
            }
            string returnjson = builder.ToString().Substring(0, builder.ToString().Length - 3);
            returnjson = returnjson + "]";
            return returnjson.Replace("\r", "").Replace("\n", "");
        }

For the update button, I utilized the following code:

cmd.Parameters.AddWithValue("@department", DropDownList2.SelectedItem.ToString());