I am using this dependency for opensearch java client version 2.1.0.
<dependency>
<groupId>org.opensearch.client</groupId>
<artifactId>opensearch-java</artifactId>
<version>2.1.0</version>
</dependency>
It is not allowing to get subaggregations from nested aggregations.
Hi @manikanta_javvadi - welcome to the community!
I’m afraid I’m not familiar enough with the topic to offer a direct answer, but in the hopes of being at least a little bit helpful, I wanted to point you to the maintainers of the repo for the opensearch-java client.
If you’re certain you’ve got your client configured correctly but there’s a feature not behaving as expected, that sounds like the perfect time to hop onto the github repo and file a bug report.
In the meantime, I’ll see if I can get someone smarter than me to address your question. 
Nate
I think this is a known bug that is about to be fixed. More info here: [BUG] Nested/Sub Aggregations are not sending back buckets containing the data · Issue #197 · opensearch-project/opensearch-java · GitHub