Hide Hamburger Menu On Specific Page (Front Page/Home Page)

You’re getting a syntax error because you forgot to close off your first if. Also, you need to close off the first php block.

<div class="mobile-nav">
    <?php if ( ! is_front_page() && ! is_home() ) { ?>
    <div class="menu-btn" id="menu-btn">
        <div></div>
        <span></span>
        <span></span>
        <span></span>
    </div>
    <?php } // <-- You forgot to put this in ?>

    <div class="responsive-menu">
    <?php
        if ( ! is_front_page() && ! is_home() ) {
        wp_nav_menu();  
        } 
    ?>
    </div>
</div>