It means you have an error somewhere else in your CSS… this is the problem:
.main-navigation:not(.is_stuck) .sticky-brand {
display: none;
remove_action('generate_after_header', 'generate_add_navigation_after_header', 5);
add_action('generate_after_header_content', function() {
generate_navigation_position();
}
);
.woocommerce-product-gallery {
display: block !important;
}
The remove_action script is PHP so needs to be removed from there. Replace the code with:
.main-navigation:not(.is_stuck) .sticky-brand {
display: none;
}
.woocommerce-product-gallery {
display: block !important;
}