порядок использования в datatable:
"ordering":'true'
"order": [columnNumber, 'orderType'],
Пример
....
"ordering":'true'
"order": [0, 'desc'],
Ваш код будет:
var oTable = $('#ks-datatable').DataTable({
'processing': false,
'serverSide': true,
"searching": false,
/*"bLengthChange": false,*/
'ajax': '{{route('invoice.index')}}',
"ordering":'true',
"order": [0, 'desc'],
columns: [
{data: 'id', name: 'id'},
{data: 'client_name', name: 'client_name'},
{data: 'invoice_number', name: 'invoice_number'},
{data: 'date', name: 'date'},
{data: 'created_at', name: 'created_at'},
{data: 'amount', name: 'amount'},
{data: 'action', name: 'action', searchable: false},
]
});