Looks like you are trying to get a link to next/previous pagination results on a single(single.php) view which it doesn’t make sense to me.
Please refer to the docs:
https://codex.wordpress.org/Function_Reference/posts_nav_link
For archive view:
For displaying next and previous pages of posts see next_posts_link()
and previous_posts_link().
For single view:
For displaying next and previous post navigation on individual posts,
see next_post_link() and previous_post_link().