why do we have hidden tests, this does not make sense, this will cause students get stuck in unknown errors without any clues. it is disgraceful, do you do this just want students get stuck intentionally so pay more money ?
It is a good question and we have bugged the course staff about this, but it is apparently some sort of limitation of the underlying Coursera grader platform and is not so easy for them to work around.
The workaround is that you ask for help here on the forums. There is a lot of prior art. Try searching for the name of the assignment in question.