Service Not Found Error when Writing Commands to ESP32 in NativeScript BLE App

I’m currently building a NativeScript app that uses BLE to connect to an ESP32 microcontroller. The microcontroller initializes a BLE server with a name, a service UUID, and characteristic UUIDs, as shown in the code snippet below. My NativeScript app successfully connects to the BLE service, but when I attempt to write commands to the characteristic, I consistently receive a service_not_found error.

#include <Arduino.h>
#include <BLEDevice.h>
#include <BLEServer.h>
#include <BLEUtils.h>
#include <BLE2902.h>
#include <ESP32Servo.h>
#include <string>

#define LOCKER_MAIN_SERVICE_UUID "3e64e8c5-6b16-4b5d-b244-5bfa29c64aa6"
#define CHARACTERISTIC_SERVO_COMMANDS_UUID "c18c2e14-0b84-4b1e-8c9e-2195c62ee5e2"

Servo myServo;
int servoPin = 5;  // Pin for servo signal

BLECharacteristic *pCharacteristic;
bool deviceConnected = false;

class LockerServerCallbacks : public BLEServerCallbacks {
  void onConnect(BLEServer* pServer) {
    deviceConnected = true;
    Serial.println("Device connected");
  }

  void onDisconnect(BLEServer* pServer) {
    deviceConnected = false;
    Serial.println("Device disconnected");
    // Restart advertising
    pServer->startAdvertising();
    Serial.println("Advertising restarted");
  }
};

class ServoCharacteristicsCallbacks : public BLECharacteristicCallbacks {
  void onWrite(BLECharacteristic *pCharacteristic) {
    // Use Arduino's String class to get the value
    String rxValue = pCharacteristic->getValue().c_str(); // Get the value as a String

    if (rxValue.length() > 0) {
      // Convert the received string to an integer (assuming single-digit commands)
      int command = rxValue[0] - '0';  // Convert ASCII character to integer

      Serial.print("Received: ");
      Serial.println(command);  // Print the received command

      // Move the servo based on the command
      moveServo(command);
    }
  }
  void moveServo(int command) {
    //moving the servo code...
  }
};

void setup() {
  Serial.begin(115200);

  // Attach the servo to the pin
  myServo.attach(servoPin);  
  myServo.write(90); // Center the servo
  delay(1000); // Wait for 2 seconds for the servo to stabilize
  
  // BLE setup
  BLEDevice::init("MyESP32Locker");
  Serial.println("Initializing MyESP32Locker");

  // Create BLE Server
  BLEServer *pServer = BLEDevice::createServer();
  pServer->setCallbacks(new LockerServerCallbacks());

  // Create BLE service
  BLEService *pService = pServer->createService(LOCKER_MAIN_SERVICE_UUID);

  // Create a BLE characteristic
  pCharacteristic = pService->createCharacteristic(
                      CHARACTERISTIC_SERVO_COMMANDS_UUID,
                      BLECharacteristic::PROPERTY_READ | BLECharacteristic::PROPERTY_WRITE
                    );

  pCharacteristic->setCallbacks(new ServoCharacteristicsCallbacks());

  // Start the service
  pService->start();
  Serial.println("BLE service started!");

  // Start advertising
  BLEAdvertising *pAdvertising = BLEDevice::getAdvertising();
  
  // Enable scan response
  pAdvertising->setScanResponse(true);
  pAdvertising->setMinPreferred(0x06);  // Functions that help with iPhone connections
  pAdvertising->setMinPreferred(0x12);

  Serial.println("Starting BLE advertising...");
  // BLEDevice::startAdvertising();

  // pAdvertising->start(); 
  pServer->startAdvertising();
  
  Serial.println("BLE advertising started, Waiting for a client connection to notify...");

}

void loop() {
  // Do nothing here

}```

Native script code:

```javascript

const Bluetooth = require("@nativescript-community/ble").Bluetooth;
const { fromObject } = require("@nativescript/core");
const createViewModel = require("@/view-models/loading-view-model").createViewModel;
const Permissions = require("nativescript-permissions"); // Correctly import the permissions module




// UUIDs for your ESP32 BLE service and characteristic (replace with actual ones)

const LOCKER_MAIN_SERVICE_UUID                          = "3e64e8c5-6b16-4b5d-b244-5bfa29c64aa6"; // Change this to a unique UUID
const CHARACTERISTIC_SERVO_COMMANDS_UUID   = "c18c2e14-0b84-4b1e-8c9e-2195c62ee5e2"; // Change this to a uniq
// Create and export the ViewModel
const viewModel = createViewModel();
const bluetooth = new Bluetooth();
let connectedPeripheral;

// Function to check and request Bluetooth permissions
function doRequestPermissions() {
    return Permissions.requestPermissions([
        android.Manifest.permission.BLUETOOTH,
        android.Manifest.permission.BLUETOOTH_ADMIN,
        android.Manifest.permission.BLUETOOTH_CONNECT,
        android.Manifest.permission.ACCESS_FINE_LOCATION,
        android.Manifest.permission.ACCESS_COARSE_LOCATION,
    ]);
}

async function onNavigatingTo(args) {
    const page = args.object;
    page.bindingContext = viewModel; // Set the binding context to the view model
    
    // TODO Check if Bluetooth on the device is enabled?!

    // Request permissions
    try {
        await doRequestPermissions();
        console.log("All permissions granted");
        
        // Start scanning for devices after permissions are granted
        scanAndConnect();
    } catch (error) {
        console.error("Error while requesting permissions: ", error);
    }
};


async function scanAndConnect() {
    
    try {
        viewModel.isConnecting = true;
        viewModel.connectionStatus = `Scanning for Locker device with service UUID ${LOCKER_MAIN_SERVICE_UUID}`;
        console.log(`Scanning for Locker device with service UUID ${LOCKER_MAIN_SERVICE_UUID}`);
        // Scan for devices with the matching SERVICE_UUID
        await bluetooth.startScanning({
            serviceUUIDs: [LOCKER_MAIN_SERVICE_UUID],
            seconds: 20,
            onDiscovered: (peripheral) => {
                if (peripheral.name && peripheral.name.startsWith("MyESP32Locker")) {
                    console.log("Discovered Locker device");
                    viewModel.connectionStatus = "Discovered Locker. Now Connecting...";
                    console.log("Peripheral object attributes:")
                    console.dir(peripheral)
                    connectedPeripheral = peripheral;
                    bluetooth.stopScanning();
                    connectToLocker(peripheral);
                }
            },
            onError:(error)=> {
                console.error("Error during BLE scan: ", error);
                viewModel.connectionStatus = "Could not find the device. Try again.";
                viewModel.isConnecting = false;
                bluetooth.stopScanning();

            }
        });
    } catch (error) {
        console.error("Error during BLE scan: ", error);
        viewModel.connectionStatus = "Error scanning for devices.";
        viewModel.isConnecting = false;
    }
}

// Function to connect to the Locker device
async function connectToLocker(peripheral) {
    try {
        console.log("Connecting to peripheral")
        viewModel.connectionStatus = "Connecting to Locker...";
        
        await bluetooth.connect({
            UUID: peripheral.UUID,
            onConnected: (peripheral) => {
                console.log("Connected successfuly to "+ peripheral.name)

                console.log("Periperhal connected with name: " + peripheral.name);

                // // the peripheral object now has a list of available services:
                // peripheral.services.forEach(function(service) {
                //     console.log("service found: " + JSON.stringify(service));
                // });< 
                
                viewModel.connectionStatus = "Connected to " + peripheral.name;
                viewModel.isConnecting = false;
            },
            onDisconnected: () => {
                console.log("Disconnected successfuly to "+ peripheral.name)
                viewModel.connectionStatus = "Disconnected from " + peripheral.name;

            }
        });
    } catch (error) {
        console.error("Error connecting to peripheral: ", error);
        viewModel.connectionStatus = "Connection failed.";
        viewModel.isConnecting = false;
    }
}

// Functions to send commands to the ESP32
async function sendCommand(command) {
    try {
        if (connectedPeripheral) {

            console.log("Sending command:")
            console.log("connectedPeripheral.UUID "+connectedPeripheral.UUID)
            console.log("LOCKER_MAIN_SERVICE_UUID "+LOCKER_MAIN_SERVICE_UUID)
            console.log("CHARACTERISTIC_SERVO_COMMANDS_UUID "+CHARACTERISTIC_SERVO_COMMANDS_UUID)
            console.log("[command] "+command)
            console.log("[command as Uint8Array] "+new Uint8Array([command]) ) // Use Uint8Array directly
            
            
            await bluetooth.write({
                peripheralUUID: connectedPeripheral.UUID,  // Use peripheralUUID instead of UUID
                serviceUUID: LOCKER_MAIN_SERVICE_UUID,
                characteristicUUID: CHARACTERISTIC_SERVO_COMMANDS_UUID,
                value: [0]
            });
            console.log(`Command ${command} sent to Locker.`);
        } else {
            console.warn("No device connected.");
        }
    } catch (error) {
        console.error("Error sending command: ", error);
    }
}

// Export functions for button taps
exports.onNavigatingTo = onNavigatingTo;

exports.onScanAndConnectTap = ()=> scanAndConnect();
exports.onOpenButtonTap = () => sendCommand(0); // Send 0 to open
exports.onCloseButtonTap = () => sendCommand(180); // Send 180 to close

Error I receive:

Sending command:
Error sending command: [BluetoothError]:service_not_found,arguments: {"peripheralUUID":"08:B6:1F:28:62:86","serviceUUID":"180e","characteristicUUID":"c18c2e14-0b84-4b1e-8c9e-2195c62ee5e2","value":[0]}

Additional Notes:

• I have verified that the characteristic and service UUIDs are correct.
• I have used the nRF Connect application on my Android phone, and it can successfully communicate with my ESP32, sending commands to the device's service with that UUID

I would greatly appreciate any help or suggestions on how to resolve this issue. Thank you in advance!

Start at sendCommand in the issue's NativeScript code and inspect the write error arguments, especially the serviceUUID actually passed. Compare that value with the ESP32 service UUID and the plugin's write API expectations; done means the write resolves against the intended characteristic and the command is observed by the ESP32.