Adding Related Articles to post

1) It’s tag__in (two underscores) 2) get_the_tags() returns an array of objects, you need IDs:

if ( $the_tags = get_the_tags() ) {
    // https://developer.wordpress.org/reference/functions/wp_list_pluck/
    $the_tags = wp_list_pluck( $the_tags, 'term_id' );
}

$rel_pst = get_posts( array(
    'tag__in'             => $the_tags,
    'exclude'             => $post->ID,
    'post_per_page'       => 4,
    'ignore_sticky_posts' => true,
    'post_type'           => array(
        'ms',
        'gnd',
        'events',
        'news_article',
        'opinions',
        'projects',
        'tenders',
        'videos',
        'products',
    ),
));

if ( $rel_pst ) {
    // Do your thing
}