There is something fishy with that specific folder it seems.
I can reproduce that error message about the Parent property, nor can I think of an explanation for that error.
Do you get the same error for all VMs ?
Can you try with another VM ?
Can you try the move to another folder ?
Can you perhaps show what is in the Parent property of that folder, by doing a
Get-Datacenter-Namecly|Get-Folder"Decom VM's"|
Select-ExpandPropertyParent|
Select*