ANSIBLE error /w JSON

The reason this is happening is because you are probably trying to run the module directly on the BIG-IP (due to a hosts: line entry that includes the BIG-IP).

This will not work though because, indeed, BIG-IP 11.6 indeed does not have a json library where this is going to work

The correct way to use the module is to delegate its usage to a machine that does have JSON, for example, localhost. For example.

- name: do a task
  bigip_facts:
      server: "my-big-ip.host.com"
      user: "admin"
      password: "mysecret"
      include: "interface,vlan"
  delegate_to: localhost

I use this method on 11.6 boxes frequently.