how to I solve [CoreBluetooth] API MISUSE: can only accept this command while in the powered on state in swift 4?

Question

I created one iOS app which search nearest bluetooth device. But when I run this app on iPhone then below error occure

[CoreBluetooth] API MISUSE: can only accept this command while in the powered on state

What can I do to solve this error and why this error occurs in the app? thanks in advance

func centralManagerDidUpdateState(_ central: CBCentralManager) {
    print(central.state)
    switch (central.state) {
    case CBManagerState.poweredOff:
       print("CoreBluetooth BLE hardware is powered off")
        break;
    case CBManagerState.unauthorized:
        print("CoreBluetooth BLE state is unauthorized")
        break

    case CBManagerState.unknown:
         print("CoreBluetooth BLE state is unknown");
        break

    case CBManagerState.poweredOn:
        print("CoreBluetooth BLE hardware is powered on and ready")
        bluetoothAvailable = true;
        break;

    case CBManagerState.resetting:
         print("CoreBluetooth BLE hardware is resetting")
        break;
    case CBManagerState.unsupported:
          print("CoreBluetooth BLE hardware is unsupported on this platform");
        break
    }
    if bluetoothAvailable == true
    {
        discoverDevices()
    }

}

func discoverDevices() {
    print("Discovering devices")
    manager?.scanForPeripherals(withServices: nil, options: nil)
    //centralManager.scanForPeripheralsWithServices(nil, options: nil)

}