Link to test case: https://pacificjustice.org/get-help/find-an-affiliate-attorney/ (try to open the from down Practice Area and you will see no data in it)
Debugger code (debug.datatables.net):

var tableAttorney = $('#table_affliate_attorney').DataTable({
                    destroy: true,
                    searching: true,
                    bLengthChange: false,
                    scrollX: true,
                    scrollY: 440,
                    autoWidth: false,
                    "language": {
                        "emptyTable": "We are sorry but there are no Affiliate Attorneys within a 150 mile radius of your requested search"
                    },
                    ajax: {
                        type: 'get',
                        url: "/wp-admin/admin-ajax.php",
                        dataType: 'json',
                        cache: false,
                        data: {
                            'action': 'get_attorney_ajax',
                            'center_lat': center_lat,
                            'center_long': center_long,
                            'state': state,
                            'city': city,
                            'zip': zip
                        }
                      
                    },
                    columns: [
                        {"data": "title"},
                        {"data": "city"},
                        {"data": "state"},
                        {"data": "zip"},
                        {"data": "distance"},
                        {
                            "data": "phone",
                            className: 'datatablePhone',
                            render: function (data) {
                                return '<a href="tel:' + data + '">' + data + '</a>';
                            }
                        },
                        {
                            "data": "email",
                            className: 'px190EM',
                            render: function (data) {
                                return '<a href="mailto:' + data + '">' + data + '</a>';
                            }
                        },
                        {
                            className: 'js-practice-area',
                            "data": "practice_area",
                        },
                        {
                            "targets": -1,
                            "data": 'email',
                            render: function (data) {
                                return "<a class='contact-lawyer' href='#' data-toggle='modal' data-target='#exampleModal' data-whatever='@mdo' data-email='"+data+"'>Contact</a>";
                            }
                        },
                        
                    ],
                    columnDefs: [
                        {"width": "150px", "targets": [0]},
                        {"width": "130px", "targets": [5]}
                    ],

                    //Select option choose practice area
                    initComplete: function () {
                        let i = 0;
                        this.api().columns().every(function () { 
                            i++;                    
                            var column = this;                              
                            if(i == 8){
                                var select = $('.individual-column-searching').on('click', 'li', function () {
                                            var val = $.fn.dataTable.util.escapeRegex($(this).text());
                                            var search_text = $.map($(this).text().split("|"), $.trim);
                                            search_text = search_text.join("|");
                                            column.search(search_text, true, false).draw();
                                        });
                            }
                            var arrayPracticeArea = [];
                                                
                            column.data().unique().sort().each(function(d,j){
                                //select.append('<li>' + d + '</li>');
                                var practiceArea = d.practice_area;
                                var jsonPacticeArea = JSON.stringify(practiceArea);
                                if (jsonPacticeArea !== undefined) {
                                    var res = $.map(jsonPacticeArea.split("|"), $.trim);
                                    
                                    for (var i = 0; i < res.length; i++) {
                                        var str = res[i];
                                        str = str.replace(/"/gi, '').trim();
                                        if (arrayPracticeArea.indexOf(str) === -1) {
                                            arrayPracticeArea.push(str);
                                        }
                                    }
                                }
                            });
                                                                
                            if (arrayPracticeArea.length !== 0) {
                                arrayPracticeArea.sort();
                                for (var k = 0; k < arrayPracticeArea.length; k++) {
                                    if (arrayPracticeArea[k].replace(/"/gi, '') === '') {
                                        continue;
                                    }
                                    $('.individual-column-searching').append('<li>' + arrayPracticeArea[k].replace(/"/gi, '') + '</li>');
                                }
                            }
                        });

Error messages shown: I am not getting the specific data from datatable that is practice_area.
Description of problem: I am trying to fetch the only practice_area from my datatable, but its simply returning undefined value, and if I just use the d parameter from column.data().unique().sort().each(function(d,j) returns all of the data that is names, cities, addresses, practice areas, etc. I just need practice_area values. Please help me out.