$wpdb->insert not working for last select option

by

The ID can’t be empty. If you have created your table and set your ID as auto increment, you don’t have to insert it, it will be generated automatically.

So all you have to do is the following:

$result = $wpdb->insert('pagos', array(
        "nombre" => $nombre, 
        "tipo" => $tipo, 
        "monto" => $monto), 
    array("%s", "%s", "%d"));