Can’t get result from sql using ajax result

I have made some changes to your code. See now if this works –

function myajax_inputtitleSubmit_func() {
    // check nonce
    $nonce = $_POST['nextNonce'];   
    if ( ! wp_verify_nonce( $nonce, 'myajax-next-nonce' ) )
        die ( 'Busted!');

    $zipcode = $_POST['zip'];
    // generate the response
    global $wpdb;
    $tablename  = "{$wpdb->prefix}levering";

    $sql    = "SELECT Levering FROM {$tablename} WHERE Zip LIKE %s";
    $result = $wpdb->get_col($wpdb->prepare($sql, $zipcode));

    $response = array();

    foreach($result as $res){
        $response[] = $res;
    }
    // response output
    echo json_encode($response);
    die();
    // IMPORTANT: don't forget to "exit"
    exit;
}