get_next_posts_link() returns no result despite available posts

get_next_posts_link() and get_previous_posts_link() are for linking between pages of a paginated post archive.

For navigation between individual posts you want get_next_post_link() and get_previous_post_link(). Note the singular “post”.

function myslug_render_post_navigation(){
    echo get_next_post_link( __( 'Previous post' ) ) . ' | ' . get_previous_post_link( __( 'Next post' ) );
}
add_action( 'genesis_after_entry', 'myslug_render_post_navigation' );

genesis();

I’ve noticed that you’ve got get_next_posts_link() and get_previous_posts_link() backwards relative to their labels, but I haven’t changed this in my example in case that was intentional.