Each license has certain usage limits, such as number of users, Application Objects, or environments. Either decrease the usage of the metric that is exceeding the limit, or upgrade your subscription. If your license becomes invalid your applications stop being available to end-users. You'll also not be allowed to make changes to the application.
Contact support for help on troubleshooting your license usage or upgrading your license. To speed up this process, be sure to include in your Activation Code (1), Audit License Information (2), and End-User Sessions usage, if available.
For more information check the OutSystems Platform licensing FAQ.