It’s really difficult to determine that without the error message generated by the code or the rest of the code that goes with this. Post a link to your repl so that the code can be debugged.
You really just need to follow the exercise instructions explicitly. Remember, .save() gets a callback and pay attention to the part about markModified().
Also, your repl was missing the code you originally posted. Start by adding logging of the err and data in your findById callback.