DocumentDB Elastic Cluster - Change Streams

Amazon DocumentDB clusters with auto-scaling enabled, including DocumentDB Elastic Clusters, do not currently support change streams. This could be why you're getting an error when attempting to run the admin command modifyChangeStreams.

You could use a non-elastic Amazon DocumentDB cluster or disable auto scaling on your Elastic Cluster if you want to use change streams. You could also use another database service that supports change streams, such as MongoDB Atlas or self-managed MongoDB.