Usually when we see this issue it's because the Cortex needs its ROBOTC firmware updated. To update the firmware, plug the Cortex into the computer and navigate to the 'Robot -> Download Firmware -> Automatically Update VEX Cortex' menu option; this will check both the Master Firmware (for the Master CPU) and the ROBOTC Firmware (for the Slave CPU) and update each one as needed.
You can also find a walkthrough of the process on our wiki
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