Javascript: initMap is not a function

Greetings again. I've been trying to load a map through Google Maps and it just does not show me. I run the console and I get the following message:

Uncaught InvalidValueError: initMap is not a function

I leave the code I'm occupying:

Mapa.php

<div id="map"></div>
<br>
<div>
    <table id="markers_table" class="table table-striped table-bordered" cellspacing="0" width="100%">
        <thead>
        <tr>
            <th>Fecha</th>
            <th>Vendedor</th>
            <th>Ubicaci&oacute;n</th>
        </tr>
        </thead>
        <tfoot>
        <tr>
            <th>Fecha</th>
            <th>Vendedor</th>
            <th>Ubicaci&oacute;n</th>
        </tfoot>
        <tbody>

        <?php $markers = Marker::listMarkers(); ?>
        <?php
        while (list(, $valor) = each($markers)) {
            echo " <tr>";
            echo "<td>" . $valor->getFecha(). "</td>";
            echo "<td>" . $valor->getVendedor() . "</td>";
            echo "<td>" . $valor->getUbicacion() . "</td>";
            echo " </tr>";
        }
        ?>
        </tbody>
    </table>
</div>
<script type="text/javascript">
    function initMap() {
        var map = new google.maps.Map(document.getElementById('map'), {
            zoom: 14,
            center: {lat: -36.8308521, lng: -73.0582368}
        });
        <?php $markers1 = Marker::listMarkers(); ?>
        <?php while (list(, $valor) = each($markers1)) {
        echo " var marker = new google.maps.Marker({";
        echo "position: {lat:" . $valor->getLat() . ",lng:" . $valor->getLng() . "},";
        echo " title: '" . $valor->getUbicacion() . "',";
        echo "map: map});";
    }
        ?>
    }
    $(document).ready(function() {
        $.extend($.fn.dataTable.defaults, {
            searching: false,
            ordering: false
        });
        $('#markers_table').DataTable({
            ordering: true,
            paging: false,
            "processing": true
        });
    });

</script>

Marker.php

class Marker
{
    private $fecha;
    private $vendedor;
    private $ubicacion;
    private $lat;
    private $lng;

    /**
     * Marker constructor.
     */
    public function __construct()
    {
    }


    /**
     * @return array
     */
    public static function listMarkers()
    {
        $markerList = array();
        try {
            $sql = ("select FormularioVenta_fecha as Fecha,
                    Vendedor_nombre as Vendedor,
                    ubicacion as Ubicacion,
                    FormularioVenta.FormularioVenta_gps as latlng
                    from FormularioVenta
                    left join Agricultor on FormularioVenta.Agricultor_id = Agricultor.Agricultor_id
                    left join Vendedor on Vendedor_id=FormularioVenta.FormularioVenta_usuario");
            $conn = connectDB();
            $result = query($conn,$sql);


            while ($row = mssql_fetch_array($result)) {
                $marker = new Marker();
                $date = date_format(new DateTime($row['Fecha']), 'd-m-y');
                $locations = explode(',', $row['latlng']);
                $marker->setFecha($date);
                $marker->setVendedor($row['Vendedor']);
                $marker->setUbicacion($row['Ubicacion']);
                $marker->setLat(trim($locations[0]));
                $marker->setLng(trim($locations[1]));
                array_push($markerList, $marker);
            }

            return ($markerList);
        } catch (Exception $e) {
            die(print_r(json_encode(), true));
        }
    }

    /**
     * @return mixed
     */
    public function getFecha()
    {
        return $this->fecha;
    }

    /**
     * @param mixed $fecha
     */
    public function setFecha($fecha)
    {
        $this->fecha = $fecha;
    }

    /**
     * @return mixed
     */
    public function getVendedor()
    {
        return $this->vendedor;
    }

    /**
     * @param mixed $vendedor
     */
    public function setVendedor($vendedor)
    {
        $this->vendedor = $vendedor;
    }

    /**
     * @return mixed
     */
    public function getUbicacion()
    {
        return $this->ubicacion;
    }

    /**
     * @param mixed $ubicacion
     */
    public function setUbicacion($ubicacion)
    {
        $this->ubicacion = $ubicacion;
    }

    /**
     * @return mixed
     */
    public function getLat()
    {
        return $this->lat;
    }

    /**
     * @param mixed $lat
     */
    public function setLat($lat)
    {
        $this->lat = $lat;
    }

    /**
     * @return mixed
     */
    public function getLng()
    {
        return $this->lng;
    }

    /**
     * @param mixed $lng
     */
    public function setLng($lng)
    {
        $this->lng = $lng;
    }


}

Call the Google API on Header.php

<script async defer
        src="https://maps.googleapis.com/maps/api/js?key=AIzaSyCFMa4pd7uMEU0NRi7dHS7YVBcFQvKG5Ow&signed_in=true&callback=initMap"></script>