This is expected behavior in vCenter Server 5.5.
This warning is displayed because the compatibility check only recognizes individual datastores. The compatibility check does not recognize datastore clusters. If Storage DRS is disabled an individual datastore in the Datastore Cluster can be selected. The compatibility checks will be performed after the datastore is selected. Once Storage DRS makes a recommendation and finds a destination datastore the appropriate checks are performed on the migration.
This only appears when the destination storage as a datastore cluster and Storage DRS enabled on the cluster.