How to redirect to post if search results only returns one post

Add this snippet to your functions.php

function redirect_the_single_post() {
    if (is_search() && is_main_query()) {
        global $wp_query;
        if ($wp_query->post_count == 1 && $wp_query->max_num_pages == 1) {
            wp_redirect( get_permalink( $wp_query->posts['0']->ID ) );
            exit;
        }
    }
} 
add_action('template_redirect', 'redirect_the_single_post' );

hope this will help you!!