By default Classic folders are available and as soon as you create your Attended Robot record a licensed is used up regardless to if the UiRobot is connected. Once you have created the number of robots matching your available licenses you will receive the error you posted above.
Similarly if you enable Modern Folder, you can Automatically create a robot for the user of attended type [Attended, Studio, & StudioX] as soon as the setting is updated. The license shows in use on the Licence page. The reason for this is that these are Named User licenses.
In order to do what you are wanting, you need licenses for Attended Concurrent User
Attended Concurrent - This type of license grants multiple users the possibility of simultaneously running their Robots, and to consume licenses only when they need to use Robots. This type of license is recommended if users are working in shifts.
No matter how many users are defined in Orchestrator, a license is consumed only when the Robot Tray or Agent Desktop is opened.
For example, you can have 50 users on 300 machines, all defined as Attended Robots in Orchestrator, but consume no licenses at all, because no one is using their Robot (the Robot Tray or Agent Desktop is not opened). If 2 users connect from the same machine, 2 licenses are consumed.