asynchronously update using php

I have two php files (index.php and insert.php) My index.php has a form that takes in three inputs from the user (activity, duration and date -- this is for a fitness app) and stores it in a database. I have written the code to insert into the database, which it successfully does, however now I want to add notifications to the site so that the user knows whether or not their input was inserted. For example, if the duration they input isn't a number or is less than 0, I want text to show on the site that notifies them to change the duration to an appropriate value. I have written the script for that and want to accomplish it asynchronously. Right now, I just want it to print out if the data was successfully added, which it does not. Not sure what I am doing incorrectly.

index.php

<script>
    $(function() {
        $('#getData').click(function(event) {
            event.preventDefault();

            $.ajax({
                type: "GET",
                url: "insert.php",
                data : { activity : $('#activityInput').val(), duration:  $('#durationInput').val(), date: $('#dateIn').val( },
                beforeSend: function(){
                }
                , complete: function(){
                }
                , success: function(html){
                    //this will add the new comment to the `comment' div
                    $("#comment").html(html);
                }
            });
        });
    });

</script>

<h1> Add input </h1>

<form action="insert.php" method="GET">
    Fitness activity:<br>
    <input type="text" name="activity" id="activityInput"><br>
    Duration of activity (hrs):<br>
    <input type="text" name="duration" id="durationInput"><br>
    Date (mm-dd-yyyy):<br>
    <input type="text" name="date" id="dateIn" ><br>
    <input type="submit" value="Submit" id="getData">
</form>


<div class='comment' ></div>;

insert.php

if(!empty($_GET["activity"]) && !empty($_GET["duration"]) &&    !empty($_GET["date"])) {
    $activity = mysqli_real_escape_string($link, $_GET["activity"]);
    $duration = mysqli_real_escape_string($link, $_GET["duration"]);
    $date = mysqli_real_escape_string($link, $_GET["date"]);

    if(is_numeric($duration) && $duration > 0){
        $sql = "INSERT INTO users (activity, dateInput, duration) VALUES ('$activity','$date','$duration')";
        $result = mysqli_query($link, $sql);

        mysqli_close($link);
        echo '<div class="comment">' ."Successfuly added" . '</div>';
    }
}