private void sendNotification(String message, String userId, String otherUserId) {
DatabaseReference tokensRef = FirebaseDatabase.getInstance().getReference(“FCMTokens”);

    tokensRef.child(otherUserId).get().addOnCompleteListener(task -> {
        if (task.isSuccessful() && task.getResult().exists()) {
            String token = task.getResult().getValue(String.class);
            if (token != null) {
                new Thread(() -> {
                    try {
                        String accessToken = AccessToken.getAccessToken();
                        if (accessToken != null) {
                            sendFCMNotification(token, message, userId, accessToken);
                        }
                    } catch (Exception e) {
                        Log.e("sendNotification", "Failed to send notification", e);
                    }
                }).start();
            } else {
                Log.e("sendNotification", "Token is null for user: " + otherUserId);
            }
        } else {
            Log.e("sendNotification", "Failed to get token for user: " + otherUserId, task.getException());
        }
    });
}

private void sendFCMNotification(String token, String message, String names, String accessToken) {
    try {

        URL url = new URL("https://fcm.googleapis.com/v1/projects/checki-fee94/messages:send");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("POST");
        conn.setRequestProperty("Authorization", "Bearer " + accessToken);
        conn.setRequestProperty("Content-Type", "application/json; UTF-8");
        conn.setDoOutput(true);

        JSONObject json = new JSONObject();
        JSONObject messageJson = new JSONObject();
        messageJson.put("token", token);

        // Add type information to data payload
        JSONObject data = new JSONObject();
        data.put("type", "message");


        JSONObject notification = new JSONObject();
        notification.put("title", "New Message from " + names);
        notification.put("body", message);



        messageJson.put("notification", notification);
        json.put("message", messageJson);

        try (OutputStream os = conn.getOutputStream()) {
            byte[] input = json.toString().getBytes("utf-8");
            os.write(input, 0, input.length);
        }

        int responseCode = conn.getResponseCode();
        if (responseCode == HttpURLConnection.HTTP_OK) {
            Log.d("sendFCMNotification", "Notification sent successfully");
        } else {
            Log.e("sendFCMNotification", "Failed to send notification. Response code: " + responseCode);
        }
    } catch (IOException | JSONException e) {
        Log.e("sendFCMNotification", "Error sending notification", e);
    }

}

Tying to send notification using HTTP v1 the tokens are already saved in the database.

THROWING THIS ERROR Failed to send notification. Response code: 400