Hi,
I got the same error at the beginning ==> Now, I'm using vCD 9.7 and when creating a new Provider VDC under the Add Storage window, I can only see storage policies that correspond to datastores that are NOT in any datastore cluster. It doesn't see any datastores that are part of a datastore cluster - exactly what the OP mentions.
==> I managed the error by adding a single datastore (not behind a cluster datastore) then only after i could see the other SP with datastore behing datastore cluster.
Did you solved your issue ? How ?