Call the app.js in your page.
<script src="{app.js}"></script>// in app.js
// Line 1
const phoneForm = document.querySelector("#phone");
// #phone should change with your phone input id. or you should attribute id as phone
// Line 26
const formSubmitButton = document.querySelector('.wpcf7-submit');
// Check your ContactForm submit button class. If it's different, change with in that.