Can you check the overlord log file for a complete error stack? It might help us to understand the issue.
From the error, it seems like the Kafka extension is not loaded in the _common/common.runtime.properties.
Also, if you would have installed 0.15 druid, then we could have utilized the unified druid console which runs on router port 8888. If the extension is not loaded, then this could have seen on the Load data page of the console. I have attached the screen-shot for the same.
We can see Amazon S3 is not highlighted because the extension is not added in the**_common/common.runtime.properties**