At the moment there is a bug with creating a new category in certain ACL setups. This prevents users to select "No parent" for new categories, so they can only assign it as child to one of the existing ones.
allow "Admin Login" for this group in the global configuration permissions
Go to the article manager, click on the options and set the following actions to allowed for the new group "Access Administration Interface" and "Create"
add the new group to Access Level "Special"
Create test user, assign to created group
login with test user
create new category, notice not possible to select "No parent" as parent
apply patch
create new category, and notice that you are now able to select "No parent" as parent
@test patch applied - test user was now able to select no parent