The VM has "stuck" open snapshots, and they cannot be consolidated because the zero free-space will not allow the snapshot removal process to run. VMware displays an error to this effect saying there is not enough room to add/delete snapshots (visible in the VMWare client).
If a snapshot removal procedure call from vRanger to the Virtual Center or ESX(i) host fails for some reason (power outage, nested "orphaned" snapshots on the VM, etc.), the snapshot may not be closed successfully, and will continue to grow on the VM. This could fill-up the datastore LUN on the ESX(i) host that hosts the VM in question. If disk space one the VMWare datastore LUN is close to zero, no new snapshots/snapshot removal procedures are possible, due to having no room to execute the consolidation procedure.
Free-up disk space on the datastore LUN in question temporarily, so that the process can run to consolidate the snapshots. Here are some suggested ways to do this:
1) Storage vMotion the VMs that are on the datastore to other datastores with more space, then consolidate the snapshots from the new location.
2) Power-off any disused, or non-critical VMs on the datastore LUN (if possible). When a VM is powered-off, the .vswp file (this is the VM's virtual pagefile) is not in use, and the datastore disk space it was occupying is freed-up and adds to the free space on the the Datastore LUN. The diskspace being used is equal to the memory assigned to the VM in the "Edit Settings" page of the VM. So, a VM with 4GB of RAM assigned to it has a 4GB pagefile. The suggested amount of free space to try to freeup to do a successful snapshot consolidation is 6GB, so try to have that much available. Once done:
1) Create a NEW snapshot on the VM.
2) When done, do a "Delete All" operation on the VM from Snapshot Manager. This could take several hours if the snapshot is large.