Looks like that exercise does not yet have an autograder.
On one hand, we need to add an autograder for this exercise. I've recorded that this exercise is missing an autograder. We'll be sure to add some checks on our next pass of improving the AP Java Course.
And on the other hand, we should not be showing the grading tab if the exercise is missing an autograder. That's just confusing. I made this fix and the changes will be reflected sometime later today.
Thanks for reporting this!