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. You may also not be allowed to make changes to the application (depending on the changes).
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) and Audit License Information (2).
For more information check the OutSystems Platform licensing FAQ.