Laravel validation

gt, gte, lt and lte are added in Laravel 5.6 and later versions, I'm guessing that must be the reason for you get the error. (It's working for me though.)

I think you can try like this

$request->validate([
    'product_price' => 'required|numeric|min:0|not_in:0',
]);

https://stackoverflow.com/questions/52397729/laravel-validating-a-number-greater-than-zero-is-failing