We removed the ability to add Admins to child Business Managers in the 2-tier Business Manager. Admins of a parent Business Manager can still add people as Employees to child Business Managers in order to have insight into those accounts.
To create other Business Managers, your business needs to obtain
BUSINESS_MANAGEMENT permission during the app review process. If your app is in development mode, you can surpass this requirement, but only to create two child businesses.
Please refer to the 2-tier Business Manager documentation for details.