This message is thrown when the user is trying to request a session but there are no instances available.  You will need to increase the capacity of your Max instances to make sure all users can get an instance.   If the Max is set the the appropriate amount a few things could be happening.  

1) The user may have just closed they session and the instance is reboot and all other instances are in use.

2) You have a session that is stuck in an on state.  If this is the case you will need to contact Frame support to take a look at this issue.