Auto add category based on a mapped tag

Hi,

I think no need to use the extension MB Relationship in your case. You can create a custom function to check if the post has a tag ID and then set the category for the post and hook this function to the action save_post. For example:

add_action( 'save_post', 'set_post_category', 10,3 );
 
function set_post_category( $post_id, $post, $update ) {
  // Only want to set if this is a new post!
  if ( $update ){
      return;
  }
   
  // Only set for post_type = post!
  if ( 'post' !== $post->post_type ) {
      return;
  }

  $posttags = get_the_tags( $post_id );
  if ( $posttags ) {
    foreach( $posttags as $tag ) {
      if( $tag->term_id == 12 ) {
        wp_set_post_categories( $post_id, [34, 56], true);
      }
    }
  }
}

where 12 is the tag ID, and 34, 56 is the category IDs.

Read more on the documentation
https://developer.wordpress.org/reference/hooks/save_post/
https://developer.wordpress.org/reference/functions/get_the_tags/
https://developer.wordpress.org/reference/functions/wp_set_post_categories/