As per this documentation about remote elasticsearch requirements for bitbucket, the latest supported buckler plugin (2.1.3) works with elasticsearch is 7.5.2 .

Also, I searched on your maven repo, and i found that you had buckler (2.1.4) which seems to work with elasticsearch 7.9.3 . However, it's not listed in the documentation mentioned previously.

Could you update the compatibility matrix in the official documentation ? 

What's the deadline to release buckler plugin compatible with Elasticsearch 7.10.0

Note, that Elasticsearch 8.0.0 is on the way.