"tinker" not defined

Step 1 : Install tinker
composer require laravel/tinker

Step 2 : Add provider class name into /config/app.php file
If you open the config/app.php file included with Laravel, you will see a providers array.
providers' => [     .
                    .
                    App\Providers\CountdownTimersServiceProvider::class,
                    Laravel\Tinker\TinkerServiceProvider::class

],