get_next_posts_page_link adds Inexistent directories

If you want links to the next and previous pages on a singular page created with <!--nextpage--> then you’re using the wrong function.

get_next_posts_page_link(), as suggested by the name, is for getting the next page of posts and is intended for use on archives.

To add the pagination links to a singular page, use wp_link_pages(), and use it inside the loop.

while (have_posts()) : the_post();
    the_content();

    wp_link_pages();
endwhile;

By default it outputs page numbers, but if you want “Next” and “Previous” links, set the next_or_number argument to next:

while (have_posts()) : the_post();
    the_content();

    wp_link_pages( array(
        'next_or_number' => 'next',
    ) );
endwhile;

See the documentation for more options for customising the output.