There are a number of things that can cause that type of error. The first step to debug it would be to see if there is any more information shown under “Show grader output”. Usually that will at least say something that gives some indication of whether it is a problem with the student’s code not being general enough or whether there is something structural wrong. Here’s a recent example in which it was some kind of structural problem with the notebook. And here’s one type of message you can get when the problem is that the code doesn’t work with the grader test cases. Unfortunately the debugging process in the latter type of case is pretty specific to the individual error. Did you manage to have a DM conversation and get a copy of the notebook and compare it? I’m not enrolled in M4ML, so I can’t help directly with that.
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