Google Map Error: REQUEST_DENIED: There was an issue performing a Directions request

how to find map route from source and destination address with autocomplete text search

my errror is this:

"DIRECTIONS_ROUTE: REQUEST_DENIED: There was an issue performing a Directions request.", stack: "Error: DIRECTIONS_ROUTE: REQUEST_DENIED:

how to solve this issue..

my code is below:

<script type="text/javascript">
    function mapLocation() {
        var source, destination;
        var directionsDisplay;
        var directionsService = new google.maps.DirectionsService();
        google.maps.event.addDomListener(window, 'load', function () {
            debugger;
            new google.maps.places.SearchBox(document.getElementById('txtSrc'));
            new google.maps.places.SearchBox(document.getElementById('txtDest'));
            directionsDisplay = new google.maps.DirectionsRenderer({ 'draggable': true });
        });
        function initialize() {
            debugger;
            directionsDisplay = new google.maps.DirectionsRenderer();
            var route = new google.maps.LatLng(25.5941, 85.1376);
            var mapOptions = {
                zoom: 10,
                center: route
            };
            map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);
            directionsDisplay.setMap(map);
            google.maps.event.addDomListener(document.getElementById('GetRoute'), 'click', GetRoute);
        }

        function GetRoute() {
            debugger;

            source = document.getElementById("txtSrc").value;
            destination = document.getElementById("txtDest").value;

            var request = {
                origin: source,
                destination: destination,
                travelMode: google.maps.TravelMode.DRIVING
            };

            directionsService.route(request, function (response, status) {
                debugger;

                if (status == google.maps.DirectionsStatus.OK) {
                    directionsDisplay.setDirections(response);
                    directionsDisplay.setMap(map);
                }

            });

            var service = new google.maps.DistanceMatrixService();
            service.getDistanceMatrix({
                origins: [source],
                destinations: [destination],
                travelMode: google.maps.TravelMode.DRIVING,
                unitSystem: google.maps.UnitSystem.METRIC,
                avoidHighways: false,
                avoidTolls: false
            }, function (response, status) {
                if (status == google.maps.DistanceMatrixStatus.OK && response.rows[0].elements[0].status != "ZERO_RESULTS") {
                    var distance = response.rows[0].elements[0].distance.text;
                    var duration = response.rows[0].elements[0].duration.text;
                    var dvDistance = document.getElementById("dvDistance");
                    dvDistance.innerHTML = "";
                    dvDistance.innerHTML += "Distance Is: " + distance + "<br />";
                    dvDistance.innerHTML += "Duration Is:" + duration;
                    alert(dvDistance.innerHTML);
                } else {
                    alert("Your Request For Distance Not Available");
                }
            });
        }
        google.maps.event.addDomListener(window, 'load', initialize);

    }
    mapLocation();
</script>