I'm sorry, this must be a really stupid question to most of you but I failed to get a solution elsewhere:(

I try to run this sketch

/**
 * The MySensors Arduino library handles the wireless radio link and protocol
 * between your home built sensors/actuators and HA controller of choice.
 * The sensors forms a self healing radio network with optional repeaters. Each
 * repeater and gateway builds a routing tables in EEPROM which keeps track of the
 * network topology allowing messages to be routed to nodes.
 *
 * Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
 * Copyright (C) 2013-2015 Sensnology AB
 * Full contributor list: https://github.com/mysensors/Arduino/graphs/contributors
 *
 * Documentation: http://www.mysensors.org
 * Support Forum: http://forum.mysensors.org
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * version 2 as published by the Free Software Foundation.
 *
 *******************************
 *
 * REVISION HISTORY
 * Version 1.0: Yveaux
 * 
 * DESCRIPTION
 * This sketch provides an example of how to implement a humidity/temperature
 * sensor using a HTU21D sensor.
 *  
 * 
 */

// Enable debug prints
#define MY_DEBUG

// Enable REPORT_BATTERY_LEVEL to measure battery level and send changes to gateway
//#define REPORT_BATTERY_LEVEL

// Enable and select radio type attached 
#define MY_RADIO_RF24
//#define MY_RADIO_RFM69
//#define MY_RS485

#include <MySensors.h>  

#define CHILD_ID_HUM  0
#define CHILD_ID_TEMP 1

static bool metric = true;

// Sleep time between sensor updates (in milliseconds)
//static const uint64_t UPDATE_INTERVAL = 60000;
//for debugging:
static const uint64_t UPDATE_INTERVAL = 5000;

#include <SparkFunHTU21D.h>
static HTU21D sensor;

#ifdef REPORT_BATTERY_LEVEL
#include <Vcc.h>
static uint8_t oldBatteryPcnt = 200;  // Initialize to 200 to assure first time value will be sent.
const float VccMin        = 1.8;      // Minimum expected Vcc level, in Volts: Brownout at 1.8V    -> 0%
const float VccMax        = 2.0*1.6;  // Maximum expected Vcc level, in Volts: 2xAA fresh Alkaline -> 100%
const float VccCorrection = 1.0;      // Measured Vcc by multimeter divided by reported Vcc
static Vcc vcc(VccCorrection); 
#endif

void presentation()  
{ 
  // Send the sketch info to the gateway
  sendSketchInfo("TemperatureAndHumidity", "1.0");

  // Present sensors as children to gateway
  present(CHILD_ID_HUM, S_HUM,   "Humidity");
  present(CHILD_ID_TEMP, S_TEMP, "Temperature");

  //metric = getControllerConfig().isMetric; //Erase later
}

void setup()
{
  while (!sensor.begin())
  {
    Serial.println(F("Sensor not detected!"));
    delay(5000);
  }
}


void loop()      
{  
  // Read temperature & humidity from sensor.
  const float temperature = float( sensor.readTemperature() ) / 100.0;
  const float humidity    = float( sensor.readHumidity() ) / 100.0;

#ifdef MY_DEBUG
  Serial.print(F("Temp "));
  Serial.print(temperature);
  Serial.print(metric ? 'C' : 'F');
  Serial.print(F("\tHum "));
  Serial.println(humidity);
#endif

  static MyMessage msgHum( CHILD_ID_HUM,  V_HUM );
  static MyMessage msgTemp(CHILD_ID_TEMP, V_TEMP);

  send(msgTemp.set(temperature, 2));
  send(msgHum.set(humidity, 2));

#ifdef REPORT_BATTERY_LEVEL
  const uint8_t batteryPcnt = static_cast<uint8_t>(0.5 + vcc.Read_Perc(VccMin, VccMax));

#ifdef MY_DEBUG
  Serial.print(F("Vbat "));
  Serial.print(vcc.Read_Volts());
  Serial.print(F("\tPerc "));
  Serial.println(batteryPcnt);
#endif

  // Battery readout should only go down. So report only when new value is smaller than previous one.
  if ( batteryPcnt < oldBatteryPcnt )
  {
      sendBatteryLevel(batteryPcnt);
      oldBatteryPcnt = batteryPcnt;
  }
#endif

  // Sleep until next update to save energy
  sleep(UPDATE_INTERVAL); 
}

but the compiler always complains about this line:

  while (!sensor.begin())
  {
    Serial.println(F("Sensor not detected!"));
    delay(5000);
  }

it says:

D:\XX\Mysensors\TempSen_HTU21D\TempSen_HTU21D.ino: In function 'void setup()':
TempSen_HTU21D:89:24: error: could not convert 'sensor.HTU21D::begin((* & Wire))' from 'void' to 'bool'
   while (!sensor.begin())
                        ^
TempSen_HTU21D:89:24: error: in argument to unary !

Does anybody know where the problem is? I also tried IF and all kinds of other stuff but it seems the problem is actually not this line but some other issue...

I'd be super glad if anybody can point me in the right direction...