Section "6. User Interface: Exercise" doesn't come up in the browser (is skipped during the "Continue to the next lesson" transition, and presents an "Empty Assignment Key" message when clicked directly in the course overview -- see attached), and doesn't allow the course to be marked as completed.
NB: The section can be still followed using the PDF guide for the course.
Can you try logging out and logging back into Pega Academy? And if that doesn't work, can you try unenrolling and re-enrolling in the course? One of those actions generally clears up a problem such as this.
Usually the in-enroll/re-enroll fixes the issue. I had the same issue with my enrollment in SAE Exercise Updates, and I resolved it by unenrolling and re-enrolling.
Although...I did let some time lapse between the two. That suggests that there might be some sort of timeout/refresh issue with your browser cache that's interfering with your seeing the correct behavior.
We'll look into it some more, but let us know if the issue appears to resolve on its own.