wrap AJAX in a jQuery function – Jquery ajax

I want to nest an AJAX request in a jQuery so I can put a time interval of 5 seconds.

The AJAX request does what it’s supposed to do, but from the moment I try to nest it I keep getting the message:

"jQuery.ajax is not a function"

How do I fix this – where do I make a mistake?

This is the (working) ajax request:

jQuery.ajax({
        url: "check.php",
        success: function(result) {
            var html = jQuery('<div>').html(result);

        var x = (html.find("div#a").html());
        console.log(x);
    }
});

this the nested code – which results in a jQuery.ajax is not a function error:

<script type="text/javascript">
    var row1 = "<?php echo $row_cnt; ?>";
    var interval = 1000; // 1000 = 1 second, 3000 = 3 seconds
    function doAjax() {
        jQuery.ajax({
            url: "check.php",
            success: function(result) {
                var html = jQuery('<div>').html(result);

                var x = (html.find("div#a").html());
                console.log(x);
            }
        });
    }
    setInterval(doAjax, interval);

</script>

When I google the problem, I get a lot of answers about the "slim – jquery libary", but I use this one:

<script src="https://code.jquery.com/jquery-3.5.0.js"></script>