It complains if a source is not specified in the configuration. According to doc :
The Kafka channel can be used for multiple scenarios:
With Flume source and sink - it provides a reliable and highly available channel for events
With Flume source and interceptor but no sink - it allows writing Flume events into a Kafka topic, for use by other apps
With Flume sink, but no source - it is a low-latency,fault tolerant way to send events from Kafka to Flume sinks such asHDFS, HBase or Solr
https://flume.apache.org/FlumeUserGuide.html
I'm interested in the scenario 3 however there is no example for that in official flume doc.
Regards