Running into error when trying register a S3 bucket as a repository for OpenSearch snapshots

Question

I'm very new to AWS OpenSearch and have been grappling with this for over a week; need help trying to register a S3 bucket as a repository but running into following error:

user/test is not authorized to perform: es:ESHttpPut with an explicit deny in an identity-based policy.

I have checked all policies under user/test and I do not see a explicit deny.

Using Python to setup repository: https://docs.aws.amazon.com/opensearch-service/latest/developerguide/managedomains-snapshot-registerdirectory.html

I have followed following article to set up Role/policy:

https://aws.amazon.com/blogs/big-data/take-manual-snapshots-and-restore-in-a-different-domain-spanning-across-various-regions-and-accounts-in-amazon-opensearch-service/

I am logging into a instance that has access to the opensearch domain setup in a vpc with fine grained access enabled, so not open to public and running the py script from there to register s3 as a repository.

What else can I check for any explicit deny? Any other documentation I can use to get a s3 repo to register? I have also mapped user/test and the backend role in the OpenSearch manage_snapshots section of the console. Not sure if the mapping is helping.

Any guidance is much appreciated.

Thanks.