Hello,
What is your version of Ansible ?
Starting version 2.4, vmware_guest is totally broken on the resource_pool parameter.
For previous versions, the default resource pool ithat could be indicated was /Resources and it was working.
(It has been introduced in 2.3)
To be honest, for the moment, you should use vsphere_guest.
Even if it old and "soon" to be depreciated (aka when vmware_guest will be working properly), it's much more stable for the moment than vmware_guest and still have more functions.