Get pages only with a specific taxonomy

Without seeing the code, it’s a bit hard…

But if you’re trying to query for a certain taxonomy when 'post_type' => 'page', make sure you

Add a registered Taxonomy to a registered Post Type. (See Codex)

In this case, you would add it to 'post_type' => 'page' since “Pages” do not support any taxonomies by default.

Again, you may have done this and the issue is something else, but that’s my best guess given th information at hand 🙂

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EDIT

So in theory, this would be the way about it using WP_Query, based on your code snippet

$args = array(
    'post_type' => 'page',
    'tax_query' => array(
        array(
            'taxonomy' => 'post_tag',
            'field'    => 'slug',
            'terms'    => 'montly',
        ),
    ),
);
$query = new WP_Query( $args );