can't be able to insert data from dataTable into database using PHP, jQuery, and Ajax?

I was trying to insert data from dataTable into a related table in the database using PHP, jQuery and AJAX. I don't know where am missing it out... Please I need you to help me solve this problem. The problem am having there is to register those courses in table.

Code is given below

PHP -- enroll.php

<?php
if(isset($_POST['register'])){
    $id = $_POST['register'];
    $matricNum = $_SESSION['matricNum'];
    $sqlregister = "INSERT INTO courseenrolls (matricNum, course) VALUES ((SELECT matricNum FROM student WHERE matricNum = '$matricNum'), (SELECT courseID FROM course WHERE courseID = '$id'))";
    $result_register = mysqli_query($db, $sqlregister);
    if($result_register){
        echo '<script >window.location.href="enroll.php"</script>';
    }
    else{
        echo '<script>alert("already register")</script>';
    } 
}
?>

coursereg.php

<?php
//coursereg.php
$db = mysqli_connect('localhost', 'root', 'password', 'asg');
$column = array("course.courseID", "course.courseCode", "course.courseName", "course.courseUnit");
$query = "
    SELECT * FROM level
    INNER JOIN course
    ON level.levelID = course.level
";
$query .= " WHERE ";
if(isset($_POST["is_level"]))
{
    $query .= "course.level = '".$_POST["is_level"]."' AND";
}


$data = array();

while($row = mysqli_fetch_array($result))
{
    $sub_array = array();
    $sub_array[] = $row["courseID"];
    $sub_array[] = $row["courseCode"];
    $sub_array[] = $row["courseName"];
    $sub_array[] = $row["courseUnit"];
    $sub_array[] = '<a href="enroll.php?register='.$row['courseID'].'" type="button" name="register" class="btn btn-success btn-xs success" id="register" onclick="return confirm(\'Are you sure?\')"">Register</a>';
    $data[] = $sub_array;
}
function get_all_data($db)
{
    $query = "SELECT * FROM course";
    $result = mysqli_query($db, $query);
    return mysqli_num_rows($result);
}

$output = array(
    "draw"              => intval($_POST["draw"]),
    "recordsTotal"      => get_all_data($db),
    "recordsFiltered"   => $number_filter_row,
    "data"              => $data
);

echo json_encode($output);

?>

jQuery and Ajax -- enroll.php

$(document).ready(function(){

    load_data();

     function load_data(is_level)
     {
        var dataTable = $('#course_data').DataTable({
        "processing": true,
        "serverSide": true,
        "order":[],
        "ajax":{
            url:"coursereg.php",
            type:"POST",
            data:{is_level:is_level}
        },
        "columsDefs":[
                {
                    "targets":[2],
                    "orderable":false,
                },
            ],

        });
    }

    $(document).on('change', '#level', function(){
        var level = $(this).val();
        $('#course_data').DataTable().destroy();
        if(level != '')
        {
            load_data(level);
        }
        else
        {
            load_data();
        }
    });

    // $(document).
});
</script>

The whole essence of it is that i want students (Tbl: Student) to register course (Tbl: Course) from the DataTable which will be inserted into courseEnrolls (Tbl: CourseEnrolls).