Unable to create group hub using API

I believe the issue is with the "parent" key, which shouldn't be necessary.  If you try it without that but still with the "parent_category" key (assuming that category exists) then it should work for you.  

For example:

{
    "grouphub":{
        "id":"shutterbugs",
        "title":"Shutterbugs",
        "description" : "test",
        "membership_type":"open",
        "conversation_styles":[
            "forum",
            "blog"
        ],
        "parent_category":{
      	    "id":"testGroups"
        }
    }
}

Alternatively, if you are using Python to make the REST API calls then you could utilize the open-source Khoros Python library I'm developing to create your group hubs, as documented here and shown in the example below.

>>> khoros.grouphubs.create(
...     'python-lovers', 'The Python Lovers Group', \
...     'This is my witty description.', \
...     open_group=True, discussion_styles=['forum', 'blog'], \
...     parent_category_id='testGroups', return_url=True)
'https://stage.example.com/t5/the-python-lovers-group/gh-p/python-lovers'

Hope this helps!