Hello @Mauricio_Montilla,
There are two lines at the beginning of each graded code cell for the autograder to identify them. If they are changed, then the error that you reported will happen. Adding them back like the image above will get rid of that error.
On the other hand, if you prefer to just get a new copy of the assignment so as to make sure all autograder-needed lines are there, you may check out B5 of the MLS FAQ.
Cheers,
Raymond