is_singular will return false on an archive page. If you wanted to check if you are on an archive for a post type, you would use is_post_type_archive( 'libri' ) , or you can create an archive-{post_type}.php file, which on this case would be archive-libri.php
References: https://codex.wordpress.org/Post_Type_Templates https://codex.wordpress.org/Conditional_Tags#A_Single_Post_Page