Sensor Si7021 / SHT31 not recognized

Hi after looking for a solution to my problem, I found this line in th mini ultra pro v3 section:
Data logging with Mini Ultra Pro Coming Soon
so I have to ask here how I solve my problem: I tried with two sensors on the pins GND, 3V3, SDA, SCL to read sensor data without luck. Do you have a solution for this (what am I doing wrong)?

/*******************************************************************************
 Mini Ultra Pro Sleep RTC Alarm
 Version: 0.10
 Date: 20-02-2017
 Company: Rocket Scream Electronics
 Author: Lim Phang Moh
 Website: www.rocketscream.com

 An example code demonstrating the ultra low sleep current capability of the 
 Mini Ultra Pro while running the on-chip built-in RTC. Example puts the Mini
 Ultra Pro into standby mode (sleep) while RTC is allowed to run in the 
 background. Alarm is set to wake the processor on the 15 s of a minute.
 This results in sleep period of 1 minute. The LED on pin 13 is set to toggle
 on every alarm match state. In sleep state with the built-in LED turn off, 
 current measured is approximately 21.0 uA.

 Requirement:
 1. Mini Ultra Pro board powered by a single cell Li-Ion/Pol 3.7V battery.

 Important Note:
 1. Upon using the SerialFlash.sleep() function, the only function that the 
 serial flash chip response to is SerialFlash.wakeup(). 

 Revision Description
 ======== ===========
 0.10 Initial public release.
*******************************************************************************/

#include <SerialFlash.h>
#include <RTCZero.h>
#include <SPI.h>
#include <RH_RF95.h>
#include <RHDatagram.h>
#include <CayenneLPP.h>

#define DEBUG

#define SERVER_ADDR 1
#define NODE_ADDR 32

#define SENSOR_SI7021
//#define SENSOR_SHT31

#define RF_FREQUENCY 868.0

#define Serial SerialUSB

// ***** CONSTANTS *****
const int radioDio0 = 2;
const int flashChipSelect = 4;
const int radioChipSelect = 5;

// Choose correct on-board radio
RH_RF95 radio(radioChipSelect, radioDio0);
RHDatagram manager(radio, NODE_ADDR);
RTCZero rtc;
CayenneLPP lpp(51);
#ifdef SENSOR_SI7021
#include <Adafruit_Si7021.h>
Adafruit_Si7021 si7021 = Adafruit_Si7021();
#elif defined SENSOR_SHT31
#include <Adafruit_SHT31.h>
Adafruit_SHT31 sht31 = Adafruit_SHT31();
#endif

/* Change these values to set the current initial time */
const byte seconds = 0;
const byte minutes = 00;
const byte hours = 10;

/* Change these values to set the current initial date */
const byte day = 20;
const byte month = 2;
const byte year = 17;

// ***** VARIABLES *****
#if defined SENSOR_SI7021 || defined SENSOR_SHT31
float tmp;
float hum;
#endif

bool match = false;
unsigned char pinNumber;
float bat;

float readVoltage()
{
  int adcReading = analogRead(A5);
  // Discard inaccurate 1st reading
  adcReading = 0;
  // Perform averaging
  for (unsigned char counter = 10; counter > 0; counter--)
  {
    adcReading += analogRead(A5);
  }
  adcReading = adcReading / 10;
  // Convert to volts and return value
  return adcReading * (4.3 / 1023.0);
}

void alarmMatch()
{
  match = true;
//#ifdef DEBUG
//  Serial.println("Alarm callback");
//#endif
}

#ifdef SENSOR_SI7021
void init_si7021()
{
#ifdef DEBUG
  SerialUSB.print("Initializing sensor Si7021: ");
#endif
  if (si7021.begin())
  {
#ifdef DEBUG
    SerialUSB.println("OK");
#endif
  }
  else
  {
#ifdef DEBUG
    SerialUSB.println("Failed");
#endif
  }
}
#endif

#ifdef SENSOR_SHT31
void init_sht31()
{
#ifdef DEBUG
  SerialUSB.print("Initializing sensor SHT31: ");
#endif
  if (sht31.begin(0x44))
  {
#ifdef DEBUG
    SerialUSB.println("OK");
#endif
  }
  else
  {
#ifdef DEBUG
    SerialUSB.println("Failed");
#endif
  }
}
#endif

#ifdef SENSOR_SI7021
void read_si7021()
{
  tmp = si7021.readTemperature();
  lpp.addTemperature(1, tmp);
#ifdef DEBUG
  if (isnan(tmp))
    tmp = 20.0;
  SerialUSB.print("Temperature: ");
  SerialUSB.print(tmp);
  SerialUSB.println("ºC");
#endif
  hum = si7021.readHumidity();
  lpp.addRelativeHumidity(2, hum);
#ifdef DEBUG
  if (isnan(hum))
    hum = 50.0;
  SerialUSB.print("Humidity: ");
  SerialUSB.print(hum);
  SerialUSB.println("%");
#endif
}
#endif

#ifdef SENSOR_SHT31
void read_sht31()
{
  tmp = sht31.readTemperature();
  lpp.addTemperature(1, tmp);
#ifdef DEBUG
  if (isnan(tmp))
    tmp = 20.0;
  SerialUSB.print("Temperature: ");
  SerialUSB.print(tmp);
  SerialUSB.println("ºC");
#endif
  hum = sht31.readHumidity();
  lpp.addRelativeHumidity(2, hum);
#ifdef DEBUG
  if (isnan(hum))
    hum = 50.0;
  SerialUSB.print("Humidity: ");
  SerialUSB.print(hum);
  SerialUSB.println("%");
#endif
}
#endif

void setup()
{
#ifdef DEBUG
  SerialUSB.begin(9600);
  delay(10000);
  SerialUSB.println("Started");

  SerialUSB.println("Initializing board");
#endif
  // Switch unused pins as input and enabled built-in pullup 
  for (pinNumber = 0; pinNumber < 23; pinNumber++)
  {
    pinMode(pinNumber, INPUT_PULLUP);
  }
  for (pinNumber = 32; pinNumber < 42; pinNumber++)
  {
    pinMode(pinNumber, INPUT_PULLUP);
  }
  pinMode(25, INPUT_PULLUP);
  pinMode(26, INPUT_PULLUP);

#ifdef DEBUG
  SerialUSB.print("Initializing radio: ");
#endif
  if (radio.init())
  {
#ifdef DEBUG
    SerialUSB.println("OK");
#endif
  }
  else
  {
#ifdef DEBUG
    SerialUSB.println("Failed");
#endif
  }  
  radio.setFrequency(RF_FREQUENCY);
  radio.setModeTx();
  radio.setTxPower(14, false); // values between 5 to 23
  manager.setThisAddress(NODE_ADDR);
  manager.setHeaderFrom(NODE_ADDR);
  radio.sleep();
 
  SerialFlash.begin(flashChipSelect);
  SerialFlash.sleep();

#ifdef DEBUG
  pinMode(LED_BUILTIN, OUTPUT);
  digitalWrite(LED_BUILTIN, LOW);
#endif

  // ***** IMPORTANT DELAY FOR CODE UPLOAD BEFORE USB PORT DETACH DURING SLEEP *****
  delay(15000);

  //***** UNCOMMENT FOR MINI ULTRA PRO VERSION 0V30 & ABOVE IF PINS ARE CONNECTED TO RADIO ****
  pinMode(6, INPUT);
  pinMode(7, INPUT);
  digitalWrite(3, HIGH);
  pinMode(3, OUTPUT);
  digitalWrite(3, HIGH);
  //*****

#if defined SENSOR_SI7021
  init_si7021();
#elif defined SENSOR_SHT31
  init_sht31();
#endif

#ifdef DEBUG
  SerialUSB.println("Initializing clock");
#endif
  // RTC initialization
  rtc.begin();
  rtc.setTime(hours, minutes, seconds);
  rtc.setDate(day, month, year);

  // RTC alarm setting on every 30 seconds, resulting in 1 minute sleep period
  rtc.setAlarmMinutes(1);
  rtc.enableAlarm(rtc.MATCH_SS);
  rtc.attachInterrupt(alarmMatch);
#ifdef DEBUG
  SerialUSB.print("Alarm set to ");
  SerialUSB.print(rtc.getAlarmMinutes());
  SerialUSB.println(" minutes");
#else
  USBDevice.detach();
#endif
  rtc.standbyMode();
}

void loop()
{
  if (match)
  {
    match = false;

// Initialize USB and attach to host (not required if not in use)
//#ifdef DEBUG
//    USBDevice.init();
//    USBDevice.attach();
//
//    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
//#else
//    // Detach USB from host (not required if not in use)
//    USBDevice.detach();
//#endif 

    lpp.reset();

#if defined SENSOR_SI7021
    read_si7021();
#elif defined SENSOR_SHT31
    read_sht31();
#endif

#ifdef DEBUG
    SerialUSB.print("Voltage: ");
#endif
    bat = readVoltage();
    lpp.addVoltage(3, bat);
#ifdef DEBUG
    SerialUSB.print(bat);
    SerialUSB.println("V");

    SerialUSB.print("Send radio packet: ");
    digitalWrite(LED_BUILTIN, HIGH);
#endif
    radio.setModeTx();
    if (manager.sendto(lpp.getBuffer(), lpp.getSize(), SERVER_ADDR))
    {
#ifdef DEBUG
      SerialUSB.println("OK");
#endif
    }
    else
    {
#ifdef DEBUG
      SerialUSB.println("Failed");
#endif
    }
    radio.sleep();
    
#ifdef DEBUG
    SerialUSB.println("Going to sleep");
    digitalWrite(LED_BUILTIN, LOW);
#endif
    rtc.standbyMode();
  }
}