Validation with Lumen.

Does this help?

Documentation: https://lumen.laravel.com/docs/master/validation

So basically this is important information for you from that documentation page at the bottom:

Lumen does not support sessions out of the box, so the $errors view variable that is available in every view in Laravel is not available in Lumen. Should validation fail, the $this->validate helper will throw Illuminate\Validation\ValidationException with embedded JSON response that includes all relevant error messages. If you are not building a stateless API that solely sends JSON responses, you should use the full Laravel framework.