Controlio licenses apply for concurrent user sessions. Usually, user sessions are equal to the number of PCs. But in some cases, the 'client limit exceeded' message may appear if you monitor 2 user sessions on 1 PC at the same time (2 users are signed in).
Please sign out from any extra Windows/macOS session(s) or make a request to add a license.
Assuming the client has 10 licenses, and 11 users. The first 10 users to connect will be monitored, and the 11th user's monitoring will be stopped due to license limit exceeded.
When one of the 10 users frees up the license (Logout of Windows/Mac account), the 11th users will start being monitored automatically.
If needed, you can always free a license, and use it on the computer of choice.