Datatable export button not showing - datatables

Question

I am working with a student management system. Where I have filtered the data using datatable. But the export button (pdf, copy, excel) of the datatable is not working properly. All other datatable options are working correctly.
<script>
$(document).ready(function(){
var dataTable = $('#example').DataTable({
'processing': true,
'serverSide': true,
'serverMethod': 'post',
'ajax': {
'url':'log_filter.php',
'data': function(data){
// Read values
var username = $('#searchByAgent').val();
var intake_name = $('#searchByIntake').val();
var tm = $('#searchByUniversity').val();
var status = $('#searchByStatus').val();
var name = $('#searchByName').val();
var fullname = $('#searchByCounsellor').val();
// Append to data
data.searchByAgent = username;
data.searchByIntake = intake_name;
data.searchByUniversity = tm;
data.searchByStatus = status;
data.searchByName = name;
data.searchByCounsellor = fullname;
}
},
'columns': [
{ data: 'status' },
{ data: 'startdate' },
{ data: 'enddate' },
{ data: 'tm' },
{ data: 'dtm' },
{ data: 'date' },
{ data: 'course_id' }
],
dom: 'Bfrtip',
buttons: [
'copyHtml5', 'excelHtml5', 'pdfHtml5', 'csvHtml5'
]
});
});
</script>