This is most likely in a scenario with a network license and multiple users. If your license permits up to 5 users, the sixth person who tries to log on will get this message. In this scenario you should wait for one of the other users to relinquish the license by closing the application. In extreme scenarios where a user fails to relinquish the license for whatever reason, their license can be revoked and the session terminated from the Sentinel Admin Control Center on the license server machine. Double check with all users of the software before taking this step as there may be a valid reason for a user to use a license for several days (e.g. running complicated analysis on a very large dataset).