oauth-private.key does not exist or is not readable

I think that this is due to Laravel Passport, you should try the following command:

php artisan passport:install

This command will create the encryption keys needed to generate secure access tokens. In addition, the command will create "personal access" and "password grant" clients which will be used to generate access tokens

Source: https://laravel.com/docs/5.4/passport

I had the same problem when I updated the composer.I generated the keys again using php artisan passport:keys and it solved the problem

I found the solution Solution: In config/app.php I had to comment these lines:

/*Laravel\Passport\PassportServiceProvider::class,
App\Providers\CodeGrantProvider::class,
Spatie\Permission\PermissionServiceProvider::class,*/

Than you need to migrate the whole database again, than uncomment this line:

Laravel\Passport\PassportServiceProvider::class,

And run php artisan passport:install my application keys weren't working so I had to do:

php artisan config:clear
php artisan key:generate
php artisan config:clear

And than I could do php artisan serve

Thanks!