It can be a little tricky to render an ACF menu item, this snippet simplifies things quite a bit.

<?php
$cta = get_field('header_cta', 'nav_menu_' . get_nav_menu_locations()['main-menu']);
if (!empty($cta['url']) && !empty($cta['title'])) :
?>
<a href="<?= esc_url($cta['url']); ?>" class="header__cta" target="<?= esc_attr($cta['target'] ?: '_self'); ?>">
<?= esc_html($cta['title']); ?>
</a>
<?php endif; ?>