The search engine of my website finds only posts and not pages, how can I solve this problem?

WordPress search results can depend on your theme or plugins as well.
Try enabling the default theme called Twenty Ninteen and then try your search.

The default behavior is to search both, posts and pages. However if your theme is changing that you can force the search of pages.

In order to search only pages in WordPress, we will need to add a PHP filter to the WordPress functions file. Open your functions.php file then copy and paste the code below. Your WordPress site will now return pages and posts in the search results.

function SearchFilter($query) {
    if ($query->is_search) {
        $query->set('post_type', array('post', 'page'));
    }
    return $query;
}
add_filter('pre_get_posts', 'SearchFilter');