My problem

problem.cpp

void isCardWrite(String data)
{
  MFRC522::MIFARE_Key key;
  for (byte i = 0; i < 6; i++) key.keyByte[i] = 0xFF;

  byte buffer[16];
  data.getBytes(buffer, 16); // A vissza adott érték,ha az 12345678-as stringet adok be:3132333435363738
  /*for (int i = 0; i < sizeof(buffer); i++)
  {
      buffer[i]=(byte)(data[i]);
  }*/

  MFRC522::StatusCode status;

  byte block = 1;
  //Serial.println(F("Authenticating using key A..."));
  status = mfrc522.PCD_Authenticate(MFRC522::PICC_CMD_MF_AUTH_KEY_A, block, &key, &(mfrc522.uid));
  if (status != MFRC522::STATUS_OK) {
    Serial.print(F("PCD_Authenticate() failed: "));
    Serial.println(mfrc522.GetStatusCodeName(status));
    return;
  }
  else Serial.println(F("PCD_Authenticate() success: "));

  // Write block
  status = mfrc522.MIFARE_Write(block, buffer, 16);
  if (status != MFRC522::STATUS_OK) {
    Serial.print(F("MIFARE_Write() failed: "));
    Serial.println(mfrc522.GetStatusCodeName(status));
    return;
  }
  else Serial.println(F("MIFARE_Write() success: "));

  mfrc522.PICC_HaltA(); // Halt PICC
  mfrc522.PCD_StopCrypto1();  // Stop encryption on PCD
}