I can't debug this code, it gives me back an error - Failed to execute 'appendChild' on 'Node': parameter 1 is not of ty

//Problem: User interaction doesn't provide desired results.
//Solution: Add iteractivity so the user can manage daily tasks. 

var taskInput = document.getElementById("new-task"); //new-task
var addButton = document.getElementsByTagName("button")[0]; //first button
var incompleteTasksHolder = document.getElementById("incomplete-tasks"); //incomplete-tasks
var completedTasksHolder = document.getElementById("completed-tasks"); //completed-tasks

//New Task List Item
var createNewTaskElement = function(taskString) {
    //Create List Item
    var listItem = document.createElement("Li");
    //input (checkbox)
    var checkBox = document.createElement("input");
    //label
    var label = document.createElement("label");
    //input (text)
    var editInput = document.createElement("input");
    //button.edit
    var editButton = document.createElement("button");
    //button.delete
    var deleteButton = document.createElement("button");
        //Each element needs modifying

        checkBox.type = "checkbox";
        editInput.type = "text";
        editButton.innerText = "Edit";
        editButton.className = "edit";
        deleteButton.innerText = "Delete";
        deleteButton.className = "delete";

        label.innerText = taskString;
        //Each element needs appending
        listItem.appendChild(checkBox);
        listItem.appendChild(label);
        listItem.appendChild(editInput);
        listItem.appendChild(editButton);
        listItem.appendChild(deleteButton);
    return listItem;
}

//Add a new task
var addTask = function() {
    console.log("Add task...");
//Create a new list item with the text from #new-task:
    var listItem = createNewTaskElement(taskInput.value);
    //Append listItem to incompleteTaskHolder
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
    taskInput.value = "";
}
//Edit an existing task
var editTask = function() {
    console.log("Edit task");

    var listItem = this.parentNode;
    var editInput = list.querySelector("input[type=text]");
    var label = listItem.querySelector("label");

    var containsClass = listItem.classList.contains("editMode");
        //if the of the parent is .editMode
        if(containsClass) {
            //Switch from .editMode
            //label text become the input's value
        label.innerText = editInput.value;
        } else { 
            //Switch to .editMode
            //input value becomes the label's text
        editInput.value = label.innerText;
        }
        //Toggle .editMode
        listItem.classList.toggle("editMode");
}

//Delete an existing task
var deleteTask = function() {
    console.log("task delete...");
    //Remove the parent list item from the ul
    var listItem = this.parentNode;
    var ul = listItem.parentNode;
    ul.removeChild(listItem);
}


//Mark a task as complete
var taskCompleted = function() {
    console.log("Task Complete...");
    //Append the task list to the #completed-tasks
    var listItem = this.parentNode;
    completedTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskIncomplete);
return listItem;
}

//Mark a task as incomplete
var taskIncomplete = function() {
    console.log("Task incomplete...");
    //Append the task list item to the #incompleted-tasks
    var listItem = this.parentNode;
    incompleteTasksHolder.appendChild(listItem);
    bindTaskEvents(listItem, taskCompleted);
//return listItem;
}

var bindTaskEvents = function(taskListItem, checkBoxEventHandler) {
    console.log("Bind list item events");
    //select taskListItem's children
    var checkbox = taskListItem.querySelector("input[type=checkbox]");
    var editButton = taskListItem.querySelector("button.edit");
    var deleteButton = taskListItem.querySelector("button.deleted");
        //bind editTask to edit Button
        editButton.onclick = editTask;

        //bind deleteTask to delete button
        window.onload = function () {
        deleteButton.onclick = deleteTask;
        }

        //bind checkBoxEventHandler to the checkbox
        checkbox.onchange = checkBoxEventHandler;
}

var ajaxRequest = function() {

    console.log("AJAX request");
}
//Set the click handler to the addTask function
addButton.addEventListener("click", addTask);
addButton.addEventListener("click", ajaxRequest);  

//cycle over incompleteTaskHolder ul list items
for(var i = 0; i < incompleteTasksHolder.children.length; i++) {
    //for each list item    
        //bind events to list item's children (taskCompleted)
        bindTaskEvents(incompleteTasksHolder.children[i], taskCompleted());
}


//cycle over incompleteTaskHolder ul list items
for(var i = 0; i < incompleteTasksHolder.children.length; i++) {
    //for each list item
        //bind events to list item's children (taskIncomplete)
        bindTaskEvents(completedTasksHolder.children[i], taskIncomplete());
}