The $ds_uuid variable, that holds the UUID of the datastore, is not correct.
Did you have the line that fetches the UUID like this ?
$ds_uuid = $ds.ExtensionData.Info.Vmfs.Uuid
Not with the quotes you mentioned earlier.
Update: just saw that it is a NFS datastore. Then forget this script, it will only work for FC-based datastores 
I thought you mentioned FC in the first post ?!?