As per http://docs.ansible.com/ansible/vmware_guest_facts_module.html (and my own testing) the vmware_guest_facts module requires a “datacenter” parameter, however, the example given on that page does not include it. To me, it doesn’t seem necessary given either the name or uuid of the VM. Even vmware_guest does not require a “datacenter” value.
In the Options table against “datacenter” it states: “Destination datacenter for the deploy operation” which makes no sense as this module does not deploy anything and so doesn’t need a destination - is this a hang-over from an older module?