javascript - How do I determine if my ajax request is failing on the client side or server side?

I'm trying to click a picture and change a database value to "0" via an onclick Javascript function. I looked at a few examples but I can't seem to make it work.

How can you use php in a javascript function

How to update data using onclick even CHECKBOX without button submit in php and mysql

Update Database from Javascript onClick By Ajax PHP

This is the index.html file

This is the update.php file.

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "dbname";



$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$num = $_POST["num"];
$id = 1;

$sql2 = "UPDATE db SET Column1='$num' WHERE id='$id'";

if ($conn->query($sql2) === TRUE) {
    echo "Updated";
} else {
    echo "Failed";
}

Would someone please point out what I'm doing wrong?