WordPress - Onclick functionality not working in <a> tags

I created a sliding sidebar menu for my WordPress site using only HTML/CSS/JavaScript. This is a tutorial from YouTube and followed everything exactly. The problem is that when I click the button, the error says "Uncaught ReferenceError: openSlideMenu is not defined in HTMLAnchorElement.onclick". For another button I get the same error. See the code below.

HTML (blog.php)

<nav class="navbar">
    <div class="side-menu">
        <span class="open-slide">
            <a href="#" onclick="openSlideMenu()">
               <svg width="30" height="30">
                <path d="M0,5 30,5" stroke="#000" stroke-width="5" />
                <path d="M0,14 30,14" stroke="#000" stroke-width="5" />
                <path d="M0,23 30,23" stroke="#000" stroke-width="5" />
               </svg> 

            </a>
        </span>
    </div>
</nav>

<div id="side-menu" class="side-nav">
    <a  href="#" class="btn-close" onclick="closeSlideMenu()">&times;</a>
        <?php get_sidebar() ?>
</div>



<script type="text/javascript">

    jQuery(document).ready(function(){


    function openSlideMenu() {
        document.getElementById('side-menu').style.width = '250px';
    }

      function closeSlideMenu() {
        document.getElementById('side-menu').style.width = '0';

    }

});

</script>

CSS

.side-nav {
    height: 100%;
    width: 0;
    position: fixed;
    z-index: 1;
    top:0;
    left:0;
    background-color: #111;
    opacity: 0.9;
    overflow-x: hidden;
    padding-top: 60px;
    transition: 0.5s;

}

.side-nav a {
    padding: 10px 10px 10px 30px;
    text-decoration: none;
    font-size:22px;
    color: #ccc;
    display: block;
    transition: 0.3s;
}


.side-nav a:hover {
    color: #fff;
}

.side-nav .btn-close {
    position: absolute;
    top: 0;
    right: 22px;
    font-size: 36px;
    margin-left: 50px;
}

The tutorial looks easy to follow and should work. I wonder if this might be related to WordPress? I do my best.