<?php
$cat = get_post_meta( get_the_ID(), 'cat_page', true );
$args = array(
'category_name' => $cat,
'meta_key' => 'niveau_chiffre',
'orderby' => 'meta_value_num', //Since we're dealing with numbers
'order' => 'ASC',
//'meta_query' => array(
// array(
//'key' => 'niveau-chiffre',
//'value' => 'niveau-chiffre', Don't give a value because we're not looking for a specific one
//'compare' => '>=', We're not comparing as such. Defaults to =
//'type' => 'NUMERIC'
//)
), //You missed a comma here I think
);
Try this.