Once you receive this error, you must restart MATLAB to complete the course. After restarting MATLAB, relaunch System Composer Onramp. Then you can skip to the appropriate section and proceed, but using the menu in the upper left of the training window (with the three bars).
You can avoid this error before you leave the Simulate and Test (Add Behavioral Models) section by navigating to the top-level architecture model before clicking Next Section. In other words, make sure that you are not currently viewing the CommunicationsBehavior or Payload reference models. The development team is aware of the issue.