Polyline does not appear on the google map - Flutter

I'm trying to add polyline between two points on the google map, but it doesn't seem to work. I write the code following by the tutorial on youtube https://youtu.be/CT6RkWL3GNE and this website https://dev.to/olayemii/adding-route-paths-polylines-between-two-points-to-google-maps-in-flutter-23o2. I use flutter_polyline_points package for creating polyline. I don't know what's wrong with my code.

This is my code.

const LatLng SOURCE_LOCATION = LatLng(13.652720, 100.493635);
const LatLng DEST_LOCATION = LatLng(13.6640896, 100.4357021);

class Direction extends StatefulWidget {
  @override
  _DirectionState createState() => _DirectionState();
}

class _DirectionState extends State<Direction> {
  Completer<GoogleMapController> mapController = Completer();

  Set<Marker> _markers = Set<Marker>();
  LatLng currentLocation;
  LatLng destinationLocation;

  Set<Polyline> _polylines = Set<Polyline>();
  List<LatLng> polylineCoordinates = [];
  PolylinePoints polylinePoints;

  @override
  void initState() {
    super.initState();
    polylinePoints = PolylinePoints();
    this.setInitialLocation();
  }

  void setInitialLocation() {
    currentLocation =
        LatLng(SOURCE_LOCATION.latitude, SOURCE_LOCATION.longitude);
    destinationLocation =
        LatLng(DEST_LOCATION.latitude, DEST_LOCATION.longitude);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Direction"),
      ),
      body: GoogleMap(
        myLocationEnabled: true,
        compassEnabled: false,
        tiltGesturesEnabled: false,
        polylines: _polylines,
        markers: _markers,
        onMapCreated: (GoogleMapController controller) {
          mapController.complete(controller);

          showMarker();
          setPolylines();
        },
        initialCameraPosition: CameraPosition(
          target: SOURCE_LOCATION,
          zoom: 13,
        ),
      ),
    );
  }

  void showMarker() {
    setState(() {
      _markers.add(Marker(
        markerId: MarkerId('sourcePin'),
        position: currentLocation,
        icon: BitmapDescriptor.defaultMarker,
      ));

      _markers.add(Marker(
        markerId: MarkerId('destinationPin'),
        position: destinationLocation,
        icon: BitmapDescriptor.defaultMarkerWithHue(90),
      ));
    });
  }

  void setPolylines() async {
    PolylineResult result = await polylinePoints.getRouteBetweenCoordinates(
        "<GOOGLE_MAPS_API_KEY_HERE>",
        PointLatLng(currentLocation.latitude, currentLocation.longitude),
        PointLatLng(
            destinationLocation.latitude, destinationLocation.longitude));

    if (result.status == 'OK') {
      result.points.forEach((PointLatLng point) {
        polylineCoordinates.add(LatLng(point.latitude, point.longitude));
      });

      setState(() {
        _polylines.add(Polyline(
            width: 10,
            polylineId: PolylineId('polyLine'),
            color: Color(0xFF08A5CB),
            points: polylineCoordinates));
      });
    }
  }
}

Seems like the issue is with your API key. I tried your code using my own API key and I was able to show the polyline (Please see screenshot below). To make it work on your end, make sure that:

• you are using a valid API key and
• Directions API is enabled in your project and
• your project is linked to a valid billing account

Please refer to this documentation to learn more.