Just to confirm, does the group already exist in the Roles and Users section or you're trying to use it to add a new user with specific roles assigned?
The cmdlet is about assigning roles to Users / Groups added, though I see that it doesn't seem to find my test group either when added already, that part let me confirm on, but can you please confirm if you're trying to add a role to an existing user / group or add a new entry in the Roles / Users section.
David Domask | Product Management: Principal Analyst