Re: Exchange OU creation issue.
kindly ensure that the organization which you are trying to create via HC panel. Its OAB, GAL, AL, RoomList should not already exist in Active Directory. Because HC also adds these objects upon creation of Exchange Org via HC panel. Therefore,make sure these objects do not already exist at back-end in Active Directory.
Another reason, could be if in the server settings under "Domain Controller Details" no "Root Organizational Unit" is entered.
I hope this piece of information shall help you in sorting out this case.