No funciona IIFE los tuve que probar de otra manera

(function(){

    const btn = document.querySelector("[data-form-btn]");

    const creacionTask = (event) => {
        const input = document.querySelector("[data-form-input]");
        const task = document.createElement("li");
        const list = document.querySelector("[data-list]");
        event.preventDefault();
        const value = input.value;
        input.value = "";
        console.log(value);

        const taskContent = document.createElement("div");
        taskContent.appendChild(checkComplete());

        const tittleTask = document.createElement("span");
        tittleTask.classList.add("task");
        tittleTask.innerHTML = value;
        taskContent.appendChild(tittleTask);


        const content = `
    <i class="fas fa-trash-alt trashIcon icon"></i>`;

        task.appendChild(taskContent);

        //list.classList.add("cardsList");
        task.classList.add("card");
        list.appendChild(task);

    };


    btn.addEventListener("click", creacionTask);

    const checkComplete = () => {
        const i = document.createElement("i");
        i.classList.add("far", "fa-check-square", "icon");
        i.addEventListener("click", complete_Task);
        return i;

    };

    const complete_Task = (event) => {
        const element = event.target;
        element.classList.toggle("fas");
        element.classList.toggle("far");
        element.classList.toggle("completeIcon");
        console.log(event.target);
    };


}());