function videotube_child_check_submit_video_tag( $errors, $data ){

    if( empty( $data['video_tag'] ) ){
        $errors->add(
            'video_tag_empty',
            esc_html__( 'Please enter video tags.', 'videotube-child' )
        );
    }

    return $errors;
}
add_filter( 'do_ajax_submit_video_errors', 'videotube_child_check_submit_video_tag', 10, 2 );