Just read the function documentation PHP: mysqli::query - Manual
mixed mysqli_query ( mysqli $link , string $query [, int $resultmode = MYSQLI_STORE_RESULT ])
The functions requires two or three parameters:
Procedural style only: A link identifier returned by mysqli_connect() or mysqli_init()
The query string.
So you have to use [EDIT: Added corrected error calls which is also missing the parameter]
mysqli_query($mysqli, "DELETE FROM tblregister WHERE id = '$id'") or die(mysqli_error($mysqli));
$mysqli->query("DELETE FROM tblregister WHERE id = '$id'") or die($mysqli->error);
I suggest to not mix procedural and object styles. Choose one and use only that. It will help to avoid such errors.
See also PHP: Dual procedural and object-oriented interface - Manual
Quote:It is possible to switch between styles at any time. Mixing both styles is not recommended for code clarity and coding style reasons.