Datastore cluster is connected to one or more host(s) that do not support storage DRS.
The datastore is connected to ESX/ESXi 4.1 or earlier hosts. According to VMware limitations, ESX/ESXi 4.1 and earlier hosts do not support storage DRS.
The datastore is connected to a host that is not running Storage I/O Control.
Ensure that all hosts associated with the datastore cluster are ESXi 5.0 or later.
Ensure that all hosts associated with the datastore cluster have Storage I/O Control enabled.
Refer to VMware Online Documentation for more information about storage DRS restrictions.