40183: FCM service account required for FCM topics

An FCM topics request was refused because FCM topics require the app's FCM credentials to be a service account, but the app is configured only with a legacy FCM server key.

What you should do

Configure the app's FCM credentials with a Firebase service account key, in place of the legacy server key. Upload the key's JSON file in the app's notification settings, or with the Ably CLI. The request then succeeds without any other change. Retrying the request does not help while the app has only a server key.

Why it happens

FCM topics are managed and sent to through the FCM HTTP v1 APIs, which authenticate with a service account. A legacy server key can't be used with them, so Ably refuses the request before contacting FCM rather than failing it later.

What you'll see

The error is reported with code 40183 and HTTP status 400. The message is unable to use FCM topics; FCM topics require the app's FCM credentials to be a service account, not a legacy server key.