This is usually caused by having a resource(s) assigned to a task on the project, but no longer on the project team tab. The CA PPM application has code to prevent issues like this coming up, but there have been some reported instances of this occurring.
Verify that all resources that are assigned to tasks on the project are listed on the "Project Team tab" within the application.
Re-staff any resources that are missing.
If you continue to have an issue, please contact CA Technical Support for further assistance.