Pardot Sandbox Setup - Unable to add business unit

I am trying to create a Pardot Sandbox in the Developer Sandbox. I am not able to select a business unit. What am I missing? 

Note: In the production Salesforce or, this is what i see. So as you can see, the 'Manager Users' button is activated, but there is no way for m to add more business units. 

I dnt know how this 'Account Status '  was enabled, when and by who ?