One of the possible reasons a clone operation can fail is because a virtual device that is listed in vCenter is inaccessible because of a permanent device loss.
For example, if the LUN ID of a newly created LUN for a clone operation matches a LUN ID that is being used by an inaccessible LUN in vCenter, the clone operation fails.
Failed to clone resources, error is Failed to Mount/Attach Disk
Refer to the VMware documentation for information.