Arduino sketch for RFM69 radio - see http://www.kittley.com/2018/04/06/blog-async-rfm69/ for more information

node.ino

// **********************************************************************************
// 
// Test RFM69 Radio.
//                                                       
// **********************************************************************************

#include <RFM69.h>              // https://www.github.com/lowpowerlab/rfm69
#include <RFM69_ATC.h>          // https://www.github.com/lowpowerlab/rfm69
#include <SPI.h>                // Included with Arduino IDE
#include <ArduinoJson.h>        // https://arduinojson.org/d
//#include <Adafruit_SleepyDog.h> // https://github.com/adafruit/Adafruit_SleepyDog

// Node and network config
#define NODEID        2    // The ID of this node (must be different for every node on network)
#define NETWORKID     100  // The network ID

// Are you using the RFM69 Wing? Uncomment if you are.
//#define USING_RFM69_WING 

// The transmision frequency of the baord. Change as needed.
#define FREQUENCY      RF69_433MHZ //RF69_868MHZ // RF69_915MHZ

// Uncomment if this board is the RFM69HW/HCW not the RFM69W/CW
#define IS_RFM69HW_HCW

// Serial board rate - just used to print debug messages
#define SERIAL_BAUD   115200

// Board and radio specific config - You should not need to edit
#if defined (__AVR_ATmega32U4__) && defined (USING_RFM69_WING)
    #define RF69_SPI_CS  10   
    #define RF69_RESET   11   
    #define RF69_IRQ_PIN 2 
    #define RF69_IRQ_NUM digitalPinToInterrupt(RF69_IRQ_PIN) 
#elif defined (__AVR_ATmega32U4__)
    #define RF69_RESET    4
    #define RF69_SPI_CS   8
    #define RF69_IRQ_PIN  7
    #define RF69_IRQ_NUM  4
#elif defined(ARDUINO_SAMD_FEATHER_M0) && defined (USING_RFM69_WING)
    #define RF69_RESET    11
    #define RF69_SPI_CS   10
    #define RF69_IRQ_PIN  6
    #define RF69_IRQ_NUM  digitalPinToInterrupt(RF69_IRQ_PIN)
#elif defined(ARDUINO_SAMD_FEATHER_M0)
    #define RF69_RESET    4
    #define RF69_SPI_CS   8
    #define RF69_IRQ_PIN  3
    #define RF69_IRQ_NUM  3
#endif

RFM69 radio(RF69_SPI_CS, RF69_IRQ_PIN, false, RF69_IRQ_NUM);

// Setup
void setup() {
  Serial.begin(115200);
  // Reset the radio
  resetRadio();
  // Initialize the radio
  radio.initialize(FREQUENCY, NODEID, NETWORKID);
  radio.promiscuous(true);
  #ifdef IS_RFM69HW_HCW
    radio.setHighPower(); //must include this only for RFM69HW/HCW!
  #endif
}

// Main loop
unsigned long previousMillis = 0;
const long sendInterval = 3000;
void loop() {

    // Receive
    if (radio.receiveDone()) {
      if (Serial) Serial.println("Message received");
      if (radio.ACKRequested()) { radio.sendACK(radio.SENDERID); }
      delay(100);
    }

    
    // Send     
    unsigned long currentMillis = millis();
    if (currentMillis - previousMillis >= sendInterval) {
      previousMillis = currentMillis;
  
      if (Serial) Serial.println("Sending");
      char payload[] = "hello from test node";
      if (radio.sendWithRetry(1, payload, sizeof(payload), 3, 200)) {
        if (Serial) Serial.println("ACK received");
      } else {
        if (Serial) Serial.println("No ACK");
      }
    }
}

// Reset the Radio
void resetRadio() {
  if (Serial) Serial.print("Resetting radio...");
  pinMode(RF69_RESET, OUTPUT);
  digitalWrite(RF69_RESET, HIGH);
  delay(20);
  digitalWrite(RF69_RESET, LOW);
  delay(500);
}

I changed mine to just radio.sendACK() which works fine