Shortly after service activation:
It may take a while for user synchronization from the Microsoft 365 tenant to be completed. Wait for a while, refresh the page and check again.
Once the user sync is completed, all tenant’s Global Admin accounts should be listed in drop-down menu and you can select which user you wish to impersonate.
However, if the drop-down menu still remains empty, please contact our Support at firstname.lastname@example.org.
Later when service has already been in use for a while:
Sometimes single permissions that have been granted for the backup application are revoked for certain reasons. This results in problems accessing the Backup portal and configuring SSO users.
You can solve this by re-granting all permissions for the Shield Cloud application. Sign in to Microsoft Entra Admin Center using the tenant's Global Admin account.
Select Applications -> Enterprise applications -> select application Shield Cloud (Application ID: facd4a2d-cbd2-4095-bfa8-ac9c819f843e)
Select Permissions under Security.
On Permissions tab, select Grant consent for [the organization]
Now the revoked permissions have been regranted and the issue with Global Admin accounts not being displayed is resolved.