In what’s possibly one of the slowest rollout of a feature in the Office 365 world, Administrative Units are not available in the Admin Center.
As I’ve already covered working with Administrative units in a blog post 3 years ago, I will not go into much detail here. Still, the fact that AUs are now respected by the Office 365 Admin Center is an important step forward, and one that should made at least some customers happy. Hopefully, a full-fledged RBAC support for Azure AD will follow soon!
In case you want more information on how AUs work in the Office 365 Admin Center, head over at the Cogmotive blog for a detailed article: https://www.cogmotive.com/blog/powershell/working-with-administrative-units-in-the-office-365-admin-center