It looks like you are attempting to update a TASK item, does your workflow have permissions to edit the task item?
Also, check to ensure that your ID variable is returning the proper data. You can insert Log to history actions within your workflow and output variables to check. This might help to pinpoint if it is a data issue or something else.
Hope this helps!
Jesse and Cassy,
Thank you for helping me with this issue. I do have permissions to update the task item. The problem was, I was using the wrong Task ID. I am all set now. Thanks to both of you!