Error "cstddef: No such file or directory" on use AzureIoTHub
Azure IoT is only supported on ESP8266 and ESP32. As stated in the README for the library:

Currently supported hardware:

ESP8266 based boards with esp8266/arduino

SparkFun Thing

Adafruit Feather Huzzah



ESP32 based boards with espressif/arduino-esp32

Adafruit HUZZAH32