Sorry, your submission was incorrect. Please try again. Parsing Python code failed: invalid syntax (<unknown>, line 21)

I tried to submit my work, but i received this message multiple times. I already tried to refresh my workplace as well even an empty submission will receive the same message. I did not revise any grading cells, I think it may have some bugs. Could you please help me to solve it?

There is an error in your code

But as i wrong the code, there is no error message, and the output value was correct as expected.
The message shows that there is something occurred in line 21. However this line 21 was blank line.

You ealier mentioned in your post that the error was syntax error. This error is caused when your code does not maintain the guidelines of python languages such as wrong indentation, etc. Can you post the screenshot of the error to better understand your problem?