How to establish a connection between 2 ES domains in Opensearch service when both of them are under the same VPC?

I am trying to establish a connection between 2 ES domains in opensearch service. Both of them are under the same VPC. I need to do so in order to perform the reindex action by taking docs from the old ES domain with ES 6.8 (remote) to new ES domain with ES 7.10 (local).

When I try to send an outbound connection request to the remote domain under Domain > Connections, it just says Validation failed (Local domain does not support VPC Endpoint connection) but doesn't specify any root cause. Are there any prerequisites for it which are not mentioned in https://docs.aws.amazon.com/opensearch-service/latest/developerguide/remote-reindex.html#remote-reindex-vpc.

Please let me know if any other info is required to answer this.