parameters do not show correctly

Question

Kotlin can`t recognize values for parameters like merchant_id , amount , description , callback_uri.

I tried to fill all keys by proper values but I got this error :

{"data":[],"errors":{"code":-9,"message":"The input params invalid, validation error.","validations":[{"merchant_id":"The merchant id field is required."},{"amount":"The amount field is required."},{"callback_url":"The callback url field is required."},{"description":"The description field is required."}]}}

here is my code :

import java.net.HttpURLConnection
import java.net.URL

fun main() {
    val url = URL("https://api.zarinpal.com/pg/v4/payment/request.json")
    val connection = url.openConnection() as HttpURLConnection
    connection.requestMethod = "POST"
    connection.setRequestProperty("Content-Type", "application/json")
    connection.setRequestProperty("Accept", "application/json")
    connection.doOutput = true

    val postData : String = """
        {
           
          "amount": 1100,
          "callback_url": "https://ali568865.ir/",
          "merchant_id": "1344b5d4-0048-11e8-94db-005056a205be",
          "back_uri": "Transaction description.",
          "metadata": {
            "mobile": "09106869409",
            "email": "[email protected]",
            "card_pan": "5022291083818920",
          }
        }
    
    """.trimIndent()

    connection.outputStream.use { outputStream ->
        outputStream.write(postData.toByteArray())
    }

    val responseCode = connection.responseCode
    if (responseCode == HttpURLConnection.HTTP_OK) {
        val response = connection.inputStream.bufferedReader().use { it.readText() }
        println(response)
    } else {
        println("Failed to make the request. Response code: $responseCode")
    }

    connection.disconnect()
}