Sum Columns don't work

Sorry, but I have this problem. I read different solutions but they don't work.
I need a sum of column in my table. This is my js code but only alert with ciao doesn't show

why? Thanks

$(document).ready(function() {
$('#example').DataTable( {
footerCallback: function () {
alert("Ciao");
var api = this.api();
$( api.table().footer() ).html(
api.column( 5, {page:'current'} ).data().sum()
);
}
} );
} );