As Milo ( and your errors ) point out: you're passing an array where a string is expected. According to [WP_Query tag parameter][1]

>Show posts associated with certain tags.

>- tag (**string**) - use tag slug.

To get around this you just need to pass a comma separated string:

    function custom_tags( $query ) {
        $query->set( 'tag', 'custom,general' );
    }
    add_action( 'pre_get_posts', 'custom_tags' );


  [1]: https://codex.wordpress.org/Class_Reference/WP_Query#Tag_Parameters