Custom metabox with image gallery upload that won’t attach images to post

Here is my function to save attachment in a field using acf plugin. It may helps

    $filename = $wp_upload_dir['path']."https://wordpress.stackexchange.com/".$img;

    // Check the type of file. We'll use this as the 'post_mime_type'.
    $filetype = wp_check_filetype( basename( $filename ), null );

    // Prepare an array of post data for the attachment.
    $attachment = array(
      'guid'           => $wp_upload_dir['url'] . "https://wordpress.stackexchange.com/" . basename( $filename ),
      'post_mime_type' => $filetype['type'],
      'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $filename ) ),
      'post_content'   => '',
      'post_status'    => 'inherit'
    );

    // Insert the attachment.
    $attach_id = wp_insert_attachment( $attachment, $filename, $post_id );

    // Make sure that this file is included, as wp_generate_attachment_metadata() depends on it.
    require_once( ABSPATH . 'wp-admin/includes/image.php' );

    // Generate the metadata for the attachment, and update the database record.
    $attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
    wp_update_attachment_metadata( $attach_id, $attach_data );

    //Save attachment id in meta
update_field( $field, $attach_id , $post_id );