Error invalid parameters with REST API

The error in question — “Invalid parameter(s): tags” and “tags[0] is not of type integer.“, means that you need to supply a list of tag IDs and not names or slugs. So for examples, 'tags' => 123 and 'tags' => [ 123 ] are both valid. (Comma-separated list is also accepted, e.g. 'tags' => '123,4,5'.)

And all that also apply to the default category taxonomy and custom taxonomies (e.g. my_tax), except that for category, you should use categories and not category. So for example, use 'categories' => 5 and not 'category' => 5.

From your comment:

is there a way for me to always use the name instead of the ID?

You can try one of these (or both for testing..):

1. You can first create the tag/category using the REST API (e.g. /wp/v2/categories for categories) and get the tag/category ID from the API response, and then use it when creating your post.

   So you’d be making two REST API requests, one for creating the tag/category, and another for creating the post.

2. On your WordPress site, you can register custom REST API fields like tags_name and categories_slug:

   // In your theme functions.php file:
   
   add_action( 'rest_api_init', 'my_register_rest_fields' );
   function my_register_rest_fields() {
       register_rest_field( 'post', 'tags_name', [
           'update_callback' => function ( $names, $post ) {
               return wp_set_post_tags( $post->ID, $names );
           }
       ] );
   
       register_rest_field( 'post', 'categories_slug', [
           'update_callback' => function ( $slugs, $post ) {
               $ids = [];
   
               foreach ( wp_parse_list( $slugs ) as $slug ) {
                   if ( $category = get_category_by_slug( $slug ) ) {
                       $ids[] = $category->term_id;
                   }
               }
   
               return ( ! empty( $ids ) ) ?
                   wp_set_post_categories( $post->ID, $ids ) : false;
           }
       ] );
   }
   

   Then when creating your post, in the API request body/data, use 'categories_slug' => 'cat-one, cat-two, etc' for categories, and 'tags_name' => 'one, two, etc' for tags. And remember, for categories, you need to use the category slug and not name.