You defined the name attribute on your input field and not the id.

So either change the attribute in the input field to this:

<input type="text" id="webAdress" />

Or adjust your JavaScript to this (MDN docu):

function myFunction() {
  var webAdress = document.getElementsByName('webAdress')[0];
  alert(webAdress);
}

All values obtained from form elements are strings. Even if the element is empty, the value is still "", not null so checking for null isn't the right approach.

Instead test for the absence of any "truthy" value as seen below.

// variables object
const el = {
  form: document.querySelector(".form"),
  input: document.querySelector(".user-input"),
  list: document.querySelector(".list"),
  date: document.querySelector(".date"),
  time: document.querySelector(".time")
};

//local storage key
const storage_key = "tasks-storage-key";

//Create ID
const createId = () => `${Math.floor(Math.random() * 10000)}-${new Date().getTime()}`;

//variable of empty array that gets new task
let taskList = [];

// function that renders task list

//function that creates new tasks with date and time
const creatTask = (task) => {
  const id = createId();
  const taskNew = el.input.value;
  const taskDate = el.date.value;
  const taskTime = el.time.value;
  const tasks = document.createElement("div");
  tasks.innerHTML = `     
      <div class="task-content">
        <div class="list-of-task">
        <div class="task" data-id="${id}">
        <input type="checkbox" class="tick">
        <div class="new-task-created">${taskNew}</div>
        <label class="due-date">${taskDate}</label>
        <label class="due-time">${taskTime}</label>
    </div>
    <div class="atcion-buttons">
        <button class="edit" data-id="">Edit</button>
        <button class="delete" data-id="">Delete</button>
    </div>
</div>`;

  taskList.push(tasks);
  console.log(taskList);
  el.list.appendChild(tasks);
  return task
};

//event listner that listens for add button.
function addTask(taskNew, taskDate, taskTime) {
  if (!taskNew) {
    alert("Please add a new Task")
  }
  if (!taskDate) {
    alert("Please add a new Task with a due date");
  }
  if (!taskTime) {
    alert("Please add a new Task with a due time");
  }
  creatTask();
}

<div class="form">
  <input class="user-input" type="text">
  <input class="date" type="date">
  <input class="time" type="time">
  <button onclick="addTask()" class="add" id="add">+</button>
</div>
<div class="list"></div>

You should prevent default action of submit button which is to submit the form.

Use Event.preventDefault();

function validateform() {
  var email = $('#txt_emailID').val();
  if (email == null || email == "") {
    alert("Email Should Be Complusory");
    return false;
  }
  var atposition = email.indexOf("@");
  var dotposition = email.lastIndexOf(".");
  if (atposition < 1 || dotposition < atposition + 2 || dotposition + 2 >= email.length) {
    alert("Please enter a valid e-mail address...!!!");
    return false;
  }
}
$(function() {
  $(document).on("click", "#submit_mail", function(e) {
    if (validateform() == false) {
      e.preventDefault();
      document.getElementById("txt_name").value = $('#txt_name').val();
      document.getElementById("txt_subject").value = $('#txt_subject').val();
    }
  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<label for="fname">Name</label>
<input type="text" id="txt_name" name="txt_name" placeholder="Your Name.." />

<label for="lname">EmailID</label>
<input type="text" id="txt_emailID" name="email" placeholder="Your EmailID.." />

<label for="lname">Subject</label>
<input type="text" id="txt_subject" name="txt_subject" placeholder="Your Subject.." />

<input type="submit" id="submit_mail" value="Send" />