The checkbox is in a td, so need to get the parent first:
$("input:checkbox").on("change", function() {
$(this).parent().next().find("label").text("TESTTTT");
});
Alternatively, find a label which has a for with the same id (perhaps more performant than reverse traversal) :
$("input:checkbox").on("change", function() {
$("label[for='" + $(this).attr('id') + "']").text("TESTTTT");
});
Or, to be more succinct just this.id:
$("input:checkbox").on("change", function() {
$("label[for='" + this.id + "']").text("TESTTTT");
});