Looks like the "volume-encryption-get-iter" API was removed in newer versions of the ONTAPI for some reason, which I believe is the reason that error is generated.
Here is ONTAPI v1.130 (an older version):
And here is ONTAPI v.1.160 (a newer version - notice how the API doesn't exist. I checked the cluster-level API too, not there either):
I haven't been able to find an alternative way to get the data other than using the "Invoke-NcSsh" command to run the ONTAP CLI commands manually. Hopefully it can be fixed in future versions.