The message indicates that all users are already logged in and you may need to ask/wait for someone to log out.
However, sometimes if a user hasn’t logged out correctly or their computer has crashed it will leave their session logged on.
You could ask all users to log out of Class and then select File/Users Logged In.
If there are any users showing as logged in but you know they are not, highlight the name and select ‘Clear’.
An Administrator of Class will need to carry out the above instructions.