Your have not a good described question, the codebase and error message should be here, not on the repository, but i can answer.

problem is in index.html :

<input type="checkbox" name="checkedbox" id="{{todo.id}}" class="taskCheckbox" value="{{todo.id}}">

after that you try in view.py:

  checkedlist = request.POST["checkedbox"]

It is famous behavior for Checkbox: if your check box not "checked", browser don't send the 'checkedbox' key.

You are simply don't has in request key 'checkedbox', you can see it yourself in debugger before the ask. :)