Greetings from ShepHertz!!!
Please make sure that Server Key that you are setting in AppHQ is correct. You will get the server key while configuring project on Google or FCM console. In FCM you need to provide legacy server key.
Please find the possible reasons for same.
Authentication Error The sender account that you're trying to use to send a message couldn't be authenticated. Possible causes are:
Authorization header missing or with invalid syntax.
Invalid project number sent as key.
Key valid but with GCM service disabled.
Request originated from a server not white listed in the Server Key IPs.
Let me know if it helps.