This is odd. One possibility is that the grader is trying to grade an un-edited version of the assignment. The grader will test against using the original filename (C1W1_Assignment, in this case), so if you made a copy and edited that file, that could be the problem.
Another possibility is if you inadvertently changed the URI path. Please take a look at this other post for more details and post back here after you try out the suggestions.