Ajax not working properly

Just saw it… …in your HTML, take the action out entirely. We’re not using the HTML to tell the form/submit button what to do, we’re using JS/AJAX for that.

<form method="POST" action="<?php echo admin_url('admin-ajax.php'); ?>" id="modal-form-ajax" autocomplete="off">

Should become:

<form method="POST" id="modal-form-ajax" autocomplete="off">

Also, instead of targeting the form, target the submit button, so the following:

$("#modal-form-ajax").submit(function(e){
     e.preventDefault();

Should become:

$("#submit-btn").on('click',function(e) {
     e.preventDefault();