You are trying to sort by “x” but WordPress doesn’t know what that is. So you either need to sort by something WP understands like “title”:
$columns['MY_CUSTOM_COLUMN'] = 'title';
Otherwise you need “x” to actually do something like such:
function my_custom_query_sort( $query ) {
if ( ! is_admin() ) {
return;
}
$orderby = $query->get( 'orderby');
if ( 'x' == $orderby ) {
$query->set( 'orderby', 'SOMETHING CUSTOM' );
}
}
add_action( 'pre_get_posts', 'my_custom_query_sort' );
But that’s not the only issue…You mention the column is a custom taxonomy. There isn’t any core function in WordPress to sort posts by taxonomy. Not to mention that each post could have multiple terms assigned to them.