As pointed out by my colleague in a former comment, the shared APM Server log messages do not line up with how an APM Server of version 7.15 would issue log messages. The logs and the error suggest to be collected from an older version of APM Server. This would also explain why you are seeing this specific error, as older APM Servers are not supporting metrics.
Please double check your server version and the configuration in your lambda app to which APM Server it is trying to connect.