Dynamically access to the validation Error messages - Laravel 5.5

I'm working on an API and i'm doing some validtions (on request) so i creat my own validations errors (validation.php) and all works fine, but i can't capture those responses to store them in my db. This is my code:

public function NullRequest(Request $request)
    { 

    //   $request->validate([
        $validator = Validator::make($request->all(), [


            'merchant_id' =>'required', //101
            'api_user' =>'required', //102
            'barcode' =>'required', //103
            'reader_name' =>'required', //104
            'branch' =>'required', //105
            'trx_type' =>'required', //106
            'secret_key' =>'required', //107
            'timestamp' =>'required', //108
            'reader_ip' =>'required', //109
            'access_token' =>'required', //110
            ]);

        if ($validator->fails()) {
            $trx_data = $validator->errors()->toArray();

            $regTrx = array(
                    'reader_name' => $request->reader_name,
                    'barcode' => $request->barcode,
                    'product_description' => 'VACIO',
                    'product_currency' => $currency[0]['iso_4712'],
                    'product_price' => 'VACIO',
                    'merchant_id' => $request->merchant_id,
                    'transaction_type' => $request->trx_type,
                    'trx_result' => $trx_data['data'][1]['Result']['Main Response: '],
                    'trx_result_extended' => $trx_data['data'][1]['Result']['Response Extended:']['Response Code: '],
                    'branch_name' => $request->branch,
                    'sector_name' => $sector[0]['sector_name'],
                    'trx_user' => $request->api_user,
                    'reader_ip' => $request->reader_ip,
                    'trx_ip' => \Request::ip(),
                    'created_at' => $timestamp,
                    'updated_at' => $timestamp
                    );

                    $this->saveTrx($regTrx);     
        }

$trx_data contains the correct rejection message

array:1 [
  "merchant_id" => array:1 [
    0 => array:2 [
      "Main Response: " => "Transaction Result Code: 1 => Conexion Rechazada - Error en la estructura del Requerimiento"
      "Response Extended:" => array:1 [
        "Response Code: " => "101 - El Campo Merchant_ID es obligatorio para este requerimiento"
      ]
    ]
  ]
]

i need to know if there is any way to dynamically access to the attribute error (merchant_id in this case) so this attribute is gonna change depending the null field in request if anyone could help me ... i'll be very thankful.. thanks in advance !

via Chebli Mohamed