Can't send Firebase Cloud Messaging with axios

Wanted to develop Web notification on Firebase Cloud Messaging. I'm using axios for requesting but it seems not to be working. It occurs an error:

Error: Request failed with status code 400

The Code:

 componentDidMount() {
    const notifications = {
      notification: [
        {
          title: "Hola!",
          body: "Hello World",
          icon: "https://xxxx.com/icon.png",
          click_action: `https://example.com/`
        }
      ],
      to: token
     };
    axios
      .post(
        "https://fcm.googleapis.com/fcm/send",
        {
          notifications
        },
        {
          headers: {
            "Content-Type": "application/json",
            Authorization:
              "key=<FCM_SERVER_KEY>"
          }
        }
      )
      .then(response => {
        console.log("response" + response);
      })
      .catch(error => {
        console.log(error);
      });