Meta description does not appear

Hi @mvneobux,
Sorry for the delay.

To output the <meta description=""> in AMP legacy templates, you could use the action amp_post_template_head:

https://github.com/Automattic/amp-wp/blob/6be1d84a07f471de3dc34f18865c0164918f27cf/templates/html-start.php#L19

For example:

/**
 * Outputs a meta description for AMP legacy templates.
 */
add_filter( 'amp_post_template_head', function() {
    ?>
    <meta description="Example description">
    <?php
} );