<button onclick="result()" type="button" name="result_submit" id="result_submit" >Submit</button>

function result(){

        $.ajax({
            url  :ajaxurl,
            type :'POST',
            action :'expense_check',
            success: function(data){
                $("#result").html(data);
            }
        });
}

<script type="text/javascript">
var ajaxurl = '<?php echo admin_url('admin-ajax.php'); ?>';
</script>

add_action( 'wp_ajax_expense', 'expense_check' );
add_action( 'wp_ajax_nopriv_expense', 'expense_check' );


function expense_check(){
include_once 'dbConnection.php';

$stmt = mysqli_stmt_init($conn);

$income = "select SUM(amount) as incomeNumber FROM wp_formdata WHERE entry_type='Income'";
$response = '';

if (! mysqli_stmt_prepare($stmt,$income)) {
     $response = '<h1 style="color:red;padding-top:5%;">SQL Error !!</h1>';
} else {
     mysqli_stmt_execute($stmt);
     $result = mysqli_stmt_get_result($stmt);
     $income_sum = mysqli_fetch_assoc($result);
     $response = "Total Income is ".$income_sum['incomeNumber'];
}

echo $response;
}

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://localhost/wordpress/wp-admin/admin-ajax.php. (Reason: CORS request did not succeed).