HTML and Form Builder is not a core component since Laravel 5, so we need to install it separately:

composer require laravelcollective/html "~5.0"

Finally in config/app.php we need to register the service provider, and the facades aliases like this:

'providers' => [
    // ...
    Collective\Html\HtmlServiceProvider::class,        
    // ...
],

'aliases' => [
   // ...
  'Form' => Collective\Html\FormFacade::class,
  'Html' => Collective\Html\HtmlFacade::class,
   // ...
],

Full docs are available on Forms & HTML