Hi all,
for anybody having the same issue.
@Jim Britt [MSFT] provided the correct answer:
Follow the instructions on: https://github.com/Azure/Enterprise-Scale/blob/main/docs/EnterpriseScale-Setup-azure.md
which state that you need to:
- Elevate Access to manage Azure resources in the directory
- Grant Access to User at root scope "/" to deploy Enterprise-Scale reference implementation
this is due to Enterprise Scale requiring permission at tenant root scope "/" to be able to configure Management Group and create/move subscription. In order to grant permission at tenant root scope "/", users in "AAD Global Administrators" group can temporarily elevate access, to manage all Azure resources in the directory.