HTTP request failed, statusCode: 403, https://maps.googleapis.com/maps/api/staticmap

I have problem when i click on Select on Map and select a location and return, there no static map appears and i get this error:

 ════════ Exception caught by image resource service
 ════════════════════════════ The following NetworkImageLoadException
 was thrown resolving an image codec: HTTP request failed, statusCode:403,
 https://maps.googleapis.com/maps/api/staticmap?center=&37.41949787164601,-122.06555999815463&zoom=13&size=600x300&maptype=roadmap&markers=color:red%7Clabel:C%7C37.41949787164601,-122.06555999815463&key="My API KEY"

Note: I enable both Maps Static API and Maps SDK for Android

This is LocationHelper class

class LocationHelper {
  static String generateLocationPreviewImage({
    double latitude,
    double longitude,
  }) {
    return 'https://maps.googleapis.com/maps/api/staticmap?center=&$latitude,$longitude&zoom=13&size=600x300&maptype=roadmap&markers=color:red%7Clabel:C%7C$latitude,$longitude&key=$GOOGLE_API_KEY';
  }

  static Future<String> getPlaceAddress(double lat, double lng) async {
    final url =
        'https://maps.googleapis.com/maps/api/geocode/json?latlng=$lat,$lng&key=$GOOGLE_API_KEY';
    final response = await http.get(Uri.parse(url));
    return json.decode(response.body)['results'][0]['formatted_address'];
  }
}

and here where i use it

import 'package:flutter/material.dart';
import 'package:location/location.dart';
import '../screens/map_screen.dart';
import '../helpers/location_helper.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

class LocationInput extends StatefulWidget {
  final Function onSelectPlace;
  LocationInput(this.onSelectPlace);

  @override
  _LocationInputState createState() => _LocationInputState();
}

class _LocationInputState extends State<LocationInput> {
  String _previewImageUrl;

  void _showPreview(double lat, double lng) {
    final staticMapImageUrl = LocationHelper.generateLocationPreviewImage(
        latitude: lat, longitude: lng);
    setState(() {
      _previewImageUrl = staticMapImageUrl;
    });
  }

  Future<void> _getCurrentLocation() async {
    try {
      final locData = await Location().getLocation();
      _showPreview(locData.latitude, locData.longitude);
      widget.onSelectPlace(locData.latitude, locData.longitude);
    } catch (error) {
      return;
    }
  }

  Future<void> _selectOnMap() async {
    final selectedLocation = await Navigator.of(context).push<LatLng>(
      MaterialPageRoute(
        fullscreenDialog: true,
        builder: (cxt) => MapScreen(
          isSelecting: true,
        ),
      ),
    );
    if (selectedLocation == null) {
      return;
    }
    // selectedLocation has longitude and latitude of the location which the user had selected
    //print(selectedLocation.latitude);
    _showPreview(selectedLocation.latitude, selectedLocation.longitude);
    widget.onSelectPlace(selectedLocation.latitude, selectedLocation.longitude);
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Container(
          height: 170,
          width: double.infinity,
          alignment: Alignment.center,
          decoration: BoxDecoration(
            border: Border.all(
              width: 1,
              color: Colors.grey,
            ),
          ),
          child: _previewImageUrl == null
              ? Text(
                  'No Loaction is Selected',
                  textAlign: TextAlign.center,
                )
              : Image.network(
                  _previewImageUrl,
                  fit: BoxFit.cover,
                  width: double.infinity,
                ),
        ),
        Row(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            FlatButton.icon(
              icon: Icon(Icons.location_on),
              label: Text('Current Location'),
              textColor: Theme.of(context).primaryColor,
              onPressed: _getCurrentLocation,
            ),
            FlatButton.icon(
              icon: Icon(Icons.map),
              label: Text('Select on Map'),
              textColor: Theme.of(context).primaryColor,
              onPressed: _selectOnMap,
            ),
          ],
        )
      ],
    );
  }
}