如何通过onClick更新字符串集

Question

我使用DynamoDB sdk对字符串集进行updateItem调用，该字符串集会收集在我的工作公告板上阅读过帖子的用户，并且我返回500错误

$readby = [
    'TableName' => $ini_array['prefix']."Bulletins",
    'Key' => [
        'BoardId' => ['S' => $_POST['boardId']], 
        'PostId' => ['S' => $_POST['postId']]
    ]
    'UpdateExpression' => ["ADD Readby: readBy"]
    'ExpressionAttributeValues' => [
        ':readBy' => ['SS' => [Employee['Email']['S']]]
    ]
];
$updateResult = $dbclient->updateItem($readbyuser);

我不知道我要去哪里错