Thanks to Azure Support, here's what happened - I log in to Azure Portal with an external user ([email protected] for example). Even though this user is a Global Admin he's not a Member of the tenant. So, either you log in with a user who's a member (@tenant.onmicrosoft.com) or you convert your user to be a Member (now, that requires you to log in as a Member admin). Here's the PowerShell script on how to convert:
Install-Module MSOnline
Connect-MsolService
Set-MsolUser -UserPrincipalName user_outlook.com#EXT#@tenant.onmicrosoft.com -UserType Member
# just to check if you became a Member user
Get-MsolUser -UserPrincipalName user_outlook.com#EXT#@tenant.onmicrosoft.com | select usertype
select
? – Jason Ye