Header not properly displaying on archive.php

You forgot to add get_header() , please try below code.

<?php get_header(); ?>
<div class="container">

<h1><?php single_cat_title();?></h1>

<?php if (have_posts()) : while(have_posts()) : the_post();?>


<div class="card mb-4">
    <div class="card-body">
    <?php if (has_post_thumbnail()):?>

<img src="<?php the_post_thumbnail_url('smallest');?>" class="img-fluid">

<?php endif;?>
    <h3><?php the_title();?></h3>
    <?php the_excerpt();?>
    <a href="<?php the_permalink();?>" class="btn btn-success">Read more</a>
    </div>
    </div>
<?php endwhile; endif;?>

</div>
<?php get_footer();?>