The error might occure for multiple reasons, you really have to provide more insight in your configuration.
- is there a reservation policy set for the reservations but not for the blueprint?
- is there a reservation policy set for the blueprint but not for the reservations?
- what do you mean by "plenty of space" in the reservation? Do you mean the bar shows space left or did you check the datastore reservations within the vra reservations one-by-one?
- what kind of clone workflow are you using for the blueprint?
- what datastore does the vm you want to clone reside on in vsphere? is the datastore available in the reservation?