When you using “laravel 5.5” downloaded project then showing this type of error, then first open terminal and got to project as below commands

step-1

$ cd project_name

step-2

$ composer update

Step-3

$ composer dump-autoload

After that your problem will be solve. If will not be solve then below commands

Step-4

$ composer require laravel/ui

After that run this commands. you project will be run successfully.

Step-5

$ php artisan serve