i.class.list.add

te copio mi codigo

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

const createTask =(evento) =>{

evento.preventDefault();
const input = document.querySelector("[data-form-input]");
const value = input.value;
input.value="";
console.log(value);
const list = document.querySelector("[data-list]")
const task = document.createElement('li');
task.classList.add("card");
console.log(task);

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

taskContent.appendChild(checkComplete());

const titleTask = document.createElement("span");

titleTask.classList.add("task");

titleTask.innerText = value;

taskContent.appendChild(titleTask)

const content = `

`;

//task.innerHTML = content; task.appendChild(taskContent);

list.appendChild(task); };

btn.addEventListener("click", createTask );

const checkComplete = ()=>{

const i = document.createElement("i");
i.classList.add("far");
i.classList.add("fa-check-square");
i.classList.add("icon");

return i;

};

No sésí ya lo resolviste pero intenta eliminando la linea de código numero 12