OK, so that means the user you’re trying to delete has posts: PostsExistError.
The suggested method of resolution here would be to review the users posts, and delete them 1-by-1, in case there are any you wish to keep by changing the user. Remember that deleting the first post in a topic deletes the topic too.
The “just do it” method would be to change delete_posts: false to delete_posts: true.