I have searched and found various results like these: auth()->user() is null in Laravel 5.2 and Auth::user() returns null
But, mine is still not working.
Thank you guys.
The problem solved here: https://laracasts.com/discuss/channels/laravel/authuser-returns-null-in-laravel-52
I had to add the stack into middleware directly (not in the group).
in /Http/kernel.php:
protected $middleware = [
\Illuminate\Foundation\Http\Middleware\CheckForMaintenanceMode::class,
\App\Http\Middleware\EncryptCookies::class,
\Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
\Illuminate\Session\Middleware\StartSession::class,
\Illuminate\View\Middleware\ShareErrorsFromSession::class,
\App\Http\Middleware\VerifyCsrfToken::class
];