Filter for Yoas SEO with Wordpress to index internal search pages. Add this in functions.php of your theme.

functions.php

//Let Google index search pages, otherwise following links will also stop
function index_search_robots ($robots) {
        if ( is_search() ) {
                return 'index,follow';
        }
        return $robots;
}
add_filter( 'wpseo_robots', 'index_search_robots' );