i fixed in this way
.....
$id_titoli[] = "$term_id"; // before was $id_titoli[] = $term_id; ===>>> the error was here
.......
$postarr = array(
'post_title' => sanitize_text_field( wp_unslash( $titolo_post ) ),
'post_status' => "publish",
'post_type' => "fight",
);
$post_id = wp_insert_post( $postarr);
.......
update_field(“match_valevole_per_titolo”,$id_titoli,$post_id);