This message indicates that not all nodes in your cluster have write access to the repository, and that is why Curator fails to make the snapshot. Curator performs a repository availability check before attempting a backup as a safety check.
If you are 100% certain that this message is in error (and it shouldn't be with a regular fs repository--occasionally s3 and azure repos are slower to respond so they give a false-negative), upgrade to Curator 3.2.0 and use the --skip-repo-validation flag.