Validation in Laravel shows controller not found error

I have a request controller CreateProductCategoryRequest.php with some rules. I want to use it in ProductCategoriesController.php so I wrote:

 public function store( CreateProductCategoryRequest $request) {
        $class = $message = '';
        --
}

But when I post the form it shows

ReflectionException in RouteDependencyResolverTrait.php line 81:

Class App\Http\Controllers\CreateProductCategoryRequest does not exist

Why this error appears ?

via Chebli Mohamed