There are some issues that we can encounter with some VMs regarding snapshots and some VM operations..
These issues can appear when you upgrade VMs HW to v9 or v10, when you migrate hosts/VMs to a new vCenter(5.0 or 5.5) when VMs have snapshots created.
Try these tips to see if they resolves your issues when you encounter the following...
When you try to revert to any snapshots we get "The operation is not supported on the object".
This one is easy to fixed, just remove the VM from the inventory and add again to vCenter. That will fix the problem.
We may use the vim-cmd vmsvc/reload vmid option here(second option on this article)
When you try to use any of the Snapshots options('Take Snapshot', 'Snapshot Manager' and 'Revert to current Snapshot') Virtual machine operations are grayed out. There is virtual machine tasks running in the background(maybe any previous task hanging).
So first you need to check which tasks are running on the VM.
Just connect to your host(ssh) and list all VMs and their vmid.
You will get all VMs and vmid associated. After you get your VM vmid Run:
If there is no task running on that virtual machine, you see output:
But even you don't see any tasks running, it has a process running (or hanging). So you should reload the vmx file(that will create a new Inventory ID (Vmid) and this should be enough to fix this issue.
vim-cmd vmsvc/reload vmid
Now the Virtual Machine should be ok and you should have all options available again.