Can't add user as admin

[nksse]

Hi

I'm trying to add a user to our Bookings portal as an Administrator. But each time the user is created as a Guest and it's the role select is disabled:

The user has a Business Premium license. I can't understand why it won't let me add them as an Admin. Other users are all Admins...

Can anyone suggest something to check?

Thanks in advance.

[#2]

Found it. The user was a 'reinstated' account and their mailbox had been converted to a shared mailbox. Once converted back to a regular mailbox they added as an Admin.