Elastic-APM Invalid index name [_license]

We are trying to get elastic-apm install (for now on our development systems).

According to Homebrew, we have the latest elasticsearch-oss (7.10.2), kibana-oss (7.10.2) and today installed apm-server-oss (which is at version 7.13.0).
Running a apm-server test output we get:

    % apm-server test output                
    elasticsearch: http://localhost:9200...
      parse url... OK
      connection...
        parse host... OK
        dns lookup... OK
        addresses: ::1, 127.0.0.1
        dial up... OK
      TLS... WARN secure connection disabled
      talk to server... ERROR Connection marked as failed because the onConnect callback failed: could not connect to a compatible version of Elasticsearch: 400 Bad Request: {"error":{"root_cause":[{"type":"invalid_index_name_exception","reason":"Invalid index name [_license], must not start with '_'.","index_uuid":"_na_","index":"_license"}],"type":"invalid_index_name_exception","reason":"Invalid index name [_license], must not start with '_'.","index_uuid":"_na_","index":"_license"},"status":400}

Because the documentation on getting APM going is somewhat obtuse, perhaps this is a configuration issue. But how to investigate further?

You need to use the same version of the stack everywhere. 7.13.0 for all products.