Erro Code: 401() ao enviar notificação FCM usando um servidor

Estou com problema ao enviar notificação usando Firebase Cloud Messaging no Android. Estou utilizando o método abaixo:

public void enviaNotificacao(String mensagem, Pedido pedido, String token) {
Sender sender = new Sender(apiKey);
Message message = new Message.Builder()
        .addData("message", mensagem)
        .addData("pedido", getGson().toJson(pedido, Pedido.class))
        .build();
try {
    Result result = sender.send(message, token, 3);

    System.out.println(result.getCanonicalRegistrationId());
    System.out.println(result.getErrorCodeName());
    System.out.println(result.getMessageId());
    System.out.println(pedido.getTokenGCM().substring(0, 10));

} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

}
Aparece o erro seguinte:

com.google.android.gcm.server.InvalidRequestException: HTTP Status Code: 401()
at com.google.android.gcm.server.Sender.makeGcmHttpRequest(Sender.java:484)
at com.google.android.gcm.server.Sender.sendNoRetry(Sender.java:215)
at com.google.android.gcm.server.Sender.send(Sender.java:182)
at com.t2ti.cardapio.Cardapio.enviaNotificacao(Cardapio.java:335)
at com.t2ti.cardapio.Cardapio.geraPedido(Cardapio.java:319)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at org.apache.axis2.rpc.receivers.RPCUtil.invokeServiceClass(RPCUtil.java:212)
at org.apache.axis2.rpc.receivers.RPCMessageReceiver.invokeBusinessLogic(RPCMessageReceiver.java:121)
at org.apache.axis2.receivers.AbstractInOutMessageReceiver.invokeBusinessLogic(AbstractInOutMessageReceiver.java:40)
at org.apache.axis2.receivers.AbstractMessageReceiver.receive(AbstractMessageReceiver.java:114)
at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:181)
at org.apache.axis2.transport.http.HTTPTransportUtils.processHTTPPostRequest(HTTPTransportUtils.java:172)
at org.apache.axis2.transport.http.AxisServlet.doPost(AxisServlet.java:146)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:648)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:617)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:668)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1527)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1484)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)

Estou usando a Chave de API da Web disponibilizado nas configurações do firebase