How to fix "Class signed does not exist" error in Laravel 5.7?

Take a look at the Laravel Documentation on Signed URLs

My guess is you are missing this entry in the $routeMiddleware array

// In app\Http\Kernel.php
/**
 * The application's route middleware.
 *
 * These middleware may be assigned to groups or used individually.
 *
 * @var array
 */
protected $routeMiddleware = [
    ...
    'signed' => \Illuminate\Routing\Middleware\ValidateSignature::class,
];