Based on the documentation: ErrorCode | Firebase
HTTP error code = 404 == UNREGISTERED
Quote:App instance was unregistered from FCM. This usually means that the token used is no longer valid and a new one must be used. This error can be caused by missing registration tokens, or unregistered tokens.
Missing Registration: If the message's target is set to token, check that the request contains a registration token.
Not registered: An existing registration token may cease to be valid in a number of scenarios, including:
- If the client app unregisters with FCM.
- If the client app is automatically unregistered, which can happen if the user uninstalls the application. For example, on iOS, if the APNS Feedback Service reported the APNS token as invalid.
- If the registration token expires (for example, Google might decide to refresh registration tokens, or the APNS token has expired for iOS devices).
- If the client app is updated but the new version is not configured to receive messages. For all these cases, remove this registration token from the app server and stop using it to send messages.
Please try with proper registration tokens.