I just found that if I simply return None, then for the tests which have a solution it would return like that. I think maybe the error msg should be changed so that it would not make us so confusing.

for some tests, I got "Solution is not a string" and "Test Passed" at the same time and for otehr tests it works normally. Can anyone tell me why this happened?

Not a suggestion. Please don't give free hints on Discourse pages, users are supposed to solve the kata by themselves.

If you don't like math or something, you just have to click next kata (top right of any kata page), or use the Practice tab to make a custom research. There are thousands of katas available, sure you will come across with something enjoyable for you.