Session Flash not Working

Hello, i want to ask something.

Anyone know why my session flash not working ? i have to try for session:put() and working fine.

app\Http\Kernel.php

protected $middleware = [
        // \App\Http\Middleware\TrustHosts::class,
        \App\Http\Middleware\TrustProxies::class,
        \Fruitcake\Cors\HandleCors::class,
        \App\Http\Middleware\PreventRequestsDuringMaintenance::class,
        \Illuminate\Foundation\Http\Middleware\ValidatePostSize::class,
        \App\Http\Middleware\TrimStrings::class,
        \Illuminate\Foundation\Http\Middleware\ConvertEmptyStringsToNull::class,
        \Illuminate\Session\Middleware\StartSession::class,
        \App\Http\Middleware\LocalizationMiddleware::class,
        \Illuminate\View\Middleware\ShareErrorsFromSession::class,
];
protected $middlewareGroups = [
        'web' => [
            \App\Http\Middleware\EncryptCookies::class,
            \Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
            \Illuminate\Session\Middleware\StartSession::class,
            // \Illuminate\Session\Middleware\AuthenticateSession::class,
            \Illuminate\View\Middleware\ShareErrorsFromSession::class,
            \App\Http\Middleware\VerifyCsrfToken::class,
            \Illuminate\Routing\Middleware\SubstituteBindings::class,
        ],

        'api' => [
            'throttle:api',
            \Illuminate\Routing\Middleware\SubstituteBindings::class,
        ],
];

** Edited: I use this for set session flash:

function checkVariant() {
...
   if($this->variant != $variant_total) {
       session()->flash("error", 'Variant need to filled.');
       return redirect()->to('/product/'.$this->product->slug);
   }
}

Sorry for my bad english.