add the post as canonical for attachment page wordpress

Here’s an (untested) example where we inject into the header tag on attachment’s pages, the canonical link of the attached post:

add_action( 'wp_head', 'wpse_attachment_parent_canonical' );

function wpse_attachment_parent_canonical()
{
    // Only target attachment's pages
    if( ! is_attachment() )
        return;

    $object = get_queried_object();

    // Make sure we're dealing with a WP_Post object
    if ( ! is_a( $object, '\WP_Post' ) )
        return;

    // Only target attachments that are attached to posts
    if( 0 == $object->post_parent )
        return;

    // Output canonical link
    printf(
        '<link rel="canonical" href="https://wordpress.stackexchange.com/questions/271590/%s" />' . PHP_EOL,
        esc_url( get_permalink( $object->post_parent ) )
    );
} 

Note that we can’t use the get_canonical_url filter here to adjust the canonical url, as it’s only applied to objects with publish post status. Attachments have inherit post status.