Google maps polyline issue in flutter

PolylinePoints polylinePoints = PolylinePoints();
PolylineResult result = await polylinePoints.getRouteBetweenCoordinates(

GOOGLE_MAPS_API_KEY,
PointLatLng(_pGooglePlex.latitude, _pGooglePlex.longitude),
PointLatLng(_pApplePark.latitude, _pApplePark.longitude),
travelMode: TravelMode.driving,
);
in this code part, it shows errors under the words getRouteBetweenCoordinates, travelMode and GOOGLE_MAPS_API_KEY. If someone can help me wwith this, would be grateful