PHP注意：未定义索引：选择消息

Question

I get always this error:

PHP Notice: Undefined index: selecting-message in /content-protection.php on line 390" and I have tried for many hours to solve this.

I hope someone can help me.

Line 390 Code:

$message = $_POST['selecting-message'];
$update  = mysqli_query($connect, "UPDATE `$table` SET enabled='$enabled', alert='$alert', message='$message' WHERE id=14");

echo '<meta http-equiv="refresh" content="0;url=content-protection">';
}