Error 404 showing

If you get that 404 when you click “Work in Browser” for that assignment, then it looks like either you accidentally renamed the notebook or perhaps it never got initialized correctly. You should be able to click “Help” and then “Lab Help → Get Latest Version” to get a fresh copy of the notebook. Here’s a thread which documents the “Get a Fresh Copy” procedure in detail. Note that it specifically deals with the case of a 404 error.