VideoTube

add_filter( 'do_ajax_submit_video_errors', function( $errors, $data ){

    $data = wp_parse_args( $data, array(
        '_thumbnail_id' =>  0
    ) );

    if( ! $data['_thumbnail_id'] ){
        $errors->add(
            'no_thumbnail',
            esc_html__( 'Thumbnail image is required', 'videotube' )
        );
    }

    return $errors;

}, 10, 2 );